Because they hate their customers and want the mouse to become junk you have to replace when the mouse goes dead. It's a finesse in planned obsolescence, same as the first time.
I have a rechargeable mouse from a no-name Chinese company, and they put the port at the nose of the mouse. It works like a wired mouse when plugged in and charging.
They have, it is especially designed to not work that way because then a lot of people would just leave it plugged in. And Apple hates that this would look like a wired mouse.
But yes that has not stopped them from doing it right on the keyboard.
The vitriol over this topic is humorous. We charge our phones and watches daily, nobody is losing their minds over that. However, asking for the mouse to be plugged in one night a month is outlandish and absurd.
The best part is that this minor issue overshadows the real issue: the ergonomics are terrible. I have large hands and simply can not hold onto the thing properly. Narrow and flat is poor design for something you need to get a grip on to slide around.
40 years since invention and outside of a few interesting edge cases like projectors, I still don't understand the purpose of a wireless mouse. It sits next to your computer full time. Pay less, get more response, much more longevity, and go wired.
My computer is on a workbench that already has a mess of wires and electronic components on it. One less wire is great. I barely game. I don't need the quick response.
I recently read a plausible reason that I hadn't thought of yet:
Apple would need to include a specific flexible cable rated for continuous movement with the mouse. If the port was in the regular spot, then people would ofc also use it wired at times. However if buyers would use regular charging cables, then the experience would both be worse and the cables might get damaged over time from bending.
I still think the main reason is simply that they value form over function, otherwise the shape would be more ergonomic, but it's another interesting factor to consider.
It's done intentionally. If you want a wired mouse, you can buy one of thousands of wired mice and use those. If you want a wireless mouse, it doesn't need to and shouldn't be plugged in while you are using it.
I think this is a pretty obtuse take. Users obviously want their mouse to remain functional when the battery dies. It's not a wireless vs wired mouse debate.