Why a ton, and not a megagram?

I think it's written 'tonne'. And you should call it metric tonne if it's not clear from the context.

Wikipedia says:

The tonne is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States customary units) and the long ton (British imperial units). The official SI unit is the megagram (symbol: Mg), a less common way to express the same amount.

https://en.wikipedia.org/wiki/Tonne

So yes, you can call it a megagramme and you'd be right. But we european people also sometimes do silly stuff and colloquially use wrong things. For example we also say it's 20 degrees celsius outside. And that's not the proper SI unit either. But that's kinda another topic.

It's typically shortened as t. So a mass of 1,000,000 kg will be referred as 1,000 t
Normally it's clear from the context and what units you are using so there is no ambiguity.
- I'm not so sure. But maybe you're right. I think I was confusing that with tonnage of a ship. But that's a whole other concept and you can't really confuse the two.
  With the 1000 t thats only because kg is a stupid SI unit and leads to the whole debacle. If there wasn't a prefix in the unit name itself, I think people would have started to use the SI unit prefixes correctly at some point instead of inventing and omitting other names to compensate.
  I think I've heard things like megatonne. For example you can say your nuclear bomb has X megaton tnt equivalent.
  A mass of a million kg should be 1 gigagram or 1 kilotonne. Not 1000t. (Edit: And not a kilotonne either, rather a mega-kilogram.)
Wait, what’s the correct SI unit for 20 degrees Celsius then? I’ve never heard anything besides that.
Edit: Nevermind, someone already asked the same question as me a bit further down. Disregard this question.

No good reason, just historical inertia and resistance to change. People stick to what they're familiar with, either the imperial system or to common metric units. Making a "metric ton" similar in size to an "imperial ton" arguably helped make it easier for some people to transition to metric.

Megagram is a perfectly cromulent unit, just like "cromulent" is a perfectly cromulent word, but people still don't use it very often. That's just how language works. People use the words they prefer, and those words become common. Maybe if you start describing things in megagrams other people will also start doing it and it will become a common part of the language. Language is organic like that, there isn't anyone making decisions on its behalf, although some people and organizations try.

I think there is even a metric inch defined as 25mm (for pipe diameters etc)
Similarly large volumes of water should be given in kl, Ml, Gl etc. instead of m^{3. Which one is bigger 2500000 m}3 or 790000 m^3? Count the zeros if you want and then tell me if using appropriate prefixes would have made it easier to tell the difference.
- If you used scientific notation or commas (or periods, depending on region) to format those numbers for human consumption, that would also make it easier.

I'm all for megagram. If nothing, it will stop the senseless people that insists on using imperial unities from confusing everybody.

WE MUST HAVE IMPERIAL UNITY!

Also, same issue as with MB and mb, you might confuse megagram with milligram

Although that's not really the reason, more like an argument to keep it this way

And you might confuse MB, megabytes, with MiB, mebibytes. MB is typically used to measure storage, and MiB typically used to measure data. There's 1000 bytes in a kilobyte, and 1024 bytes in a kibibyte.
- I still use mb and kb as 1024 instead of 1000, because I prefer to not have units switched around from under me. 2^16 will always address 64kb, not 65.
Mega mean 1024 of something right?
- Mega is a million. Kilo is a thousand. 1024 in kilobytes comes from powers of 2 which are more natural in addressing computer memory

More-common terms in any language tend to be shorter or at least less standard/more irregular.

i do see Mg instead of t in publications sometimes

Two relevant details:

The OG metric system (from the XVIII century) had no prefix for 10⁶. "Mega-" would be only formally acknowledged by the SI in 1960.
The ton units (yup, plural) backtrack all the way to a volume unit from the Middle Ages, the amount of liquid that you'd be able to put in a big arse cask

Based on those two things, I think that the ton was standardised to 10⁶g considerably before the name "megagram" had the chance to appear, to the point that it became the default name across languages.

I don't know the English name for the cask [EDIT: "tun" acc. to @theplanlessman@feddit.uk ], but in Portuguese it's "tonel". From that "tonelada" (the unit). It used to be 800kg before the metric system though.

FYI the English name of that cask is "tun".
- Thanks for the info. (To be honest I couldn't be bothered to look for it.)

I brought a shit ton of tacos. Or I have supplied us with a faecal megagram of tacos. You be the judge.

I am shocked by how well your latter example emphasizes an extremely large quantity of tacos.
I vote for that one.
- The funny thing is that one taco makes a faecal megagram, if it's the right kind of delicious, filthy taco.
An imperial shit ton or a metric shit ton?

I've never heard of gigameter or gigalitre

Because "ton" was an established amount in trade and shipping (though with significant local variations), that was later adjusted to fit into the metric system and standardized. Hence why people specify "metric ton". There was simply no need for people to change their terminology when they already had good monosyllable.

Similar to "mile" which in metric countries were brought into the standard and defined to be 10,000 meters. While these days kilometers are almost always used for long distance in all official uses, people's habits are still to talk about "miles" when describing how far something is to travel. E.g. "I live roughly 2 miles from town" flows better than "I live 22 kilometers from town".

Edit: Recent example use of metric mile: https://www.nrk.no/tromsogfinnmark/kirkenes-if-og-norild-il-ma-reise-100-mil-for-a-spill-hjemmekamp-i-fotball-nm-1.16338078

Metric citizin here, nobody uses "miles" here. We just go with the metric system:
1000 metres = 1 kilometre
We use the wording from your example "i live 22 kilometres from town"
- That's fine and I believe that is true for your country or region. Though remember there are other metric countries. E.g. the nordics still stick to it.
  https://sv.wikipedia.org/wiki/Mil
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If you think about it, tonne is actually a better base unit than grams, because it aligns better with the cubic metre (1m^3 = (approx.) 1 tonne of water.)

So really, I would ask why kilograms and milligrams, and not millitonnes and microtonnes?

I can picture it now. I weigh 70 millitonnes.

The official unit name is megagram. Ton is just used much more commonly.

Why a feet and not a fivger ?

Personally I think it’s irritating to name units with millions and billions in the names.

Imagine everything was built up from microns and we had to say “mega micron” for milimeter. And centi-mega-micron for centimeter and mega-mega-micron for meters. It gets silly and your unit names become formulas instead of highly memorable and intuitive concepts.

Once per order of magnitude you should just re-root the units with more unique names. Keep the conversions metric and clean but give “ton” and “gram” their own names because they live in different order of magnitude scales.

There are about a million reasons.

Never heard anyone use megameters either. They either stay on kilometer, or switch to miles. And miles mean different things from one place to the next.

Huh? Why would you switch to miles from kilometers?
And IMHO megameters aren't used that often because there is rarely anything useful to measure with it. Using a different unit makes you lose your sense of scale (e.g. the earth has a radius of ~7000km, not 7Mm) and for astronomy megameters aren't big enough most of the time (and you might as well use lightseconds/years because gigameters give no real intuition of scale).
Megameters are somewhat common in astronomy, for example when describing low orbital hights.
- Really? I would have though that they would use the scientific notation in meters, so that the numbers are explicitly clear.
Never heard anyone use megameters either.
I've used it in one specific situation - Physics classes in my freshman Chemistry uni year. The same professor would also use megagrams for weight.
It was mostly so the students got a bit more comfortable converting units back and forth, specially past the 10³~10⁻³ range.
- Weird, I've never used anything other than unit*10^n on physics. it's just simpler to operate. 1e3m is 1km, 1e6g a megagram. When working on science, I much prefer the scientific notation.