Why are there so few open source weather apps for iOS, compared to the very many that are on Android?

To make apps for Android you can take a 15 years old PC from the dumpster and a 5 years old smartphone that your cousin threw away and you're done. No other payment required.

To make apps for iOS you need to have a supported Mac and you need to have a supported iPhone. The OS upgrade treadmill means you need to buy a new one of both every 5-6 year (or used every 2-3 years). Finally, you need to pay a yearly $100 development subscription forever. (When you stop paying, your apps are unpublished)

Also: on Android you share the source and anyone can compile it even a decade after release. On iOS compiling old source is much more difficult as you probably need to change or fix to the updated apis

Android studio in a 15 year old pc sounds PAINFUL.
- Android studio on a modern computer is painful
  
  Also on Android, Google decides what models of phones are able to use the Play Store. So unless you get into a third party store, or expect your users to side-load, that old smartphone might not be so useful.
Yeah, subscription cost has led to apps that would be free on Android being paid on Apple or opting out of being available all together. Not just on iOS but MacOS too, and opting for being available outside the Mac store.
Android studio is a very heavy peace of garbage! you need modern hardware to get somewhere.
- Can use an ancient version of eclipse if you really want.
  
  The point btw is that there's isn't a marketing department that artificially decides that your computer is too old to run a specific application and you need to buy a new one

I've made some apps in my life and one time I made something for myself and simply skipped apple because it isn't worth the headache.

For starters you'll need an OSX VM, then pay 100 eur a year for a dev account, and then manage those god awful provisioning profiles.

Finally, side loading wasn't possible back then and their store is a black box and follows strange arbitrary American centric rules.

Edit: Also I should add that in order to get my hands on an OSX iso ten years ago I had to find someone with an apple computer to be able to purchase the OS with my account. And apparently this is a legal gray zone.

These days it's a bit easier because you can use a cloud build provider and just stuff the provisioning profiles in there, for example if you use Unity3d.

In addition to the reasons already mentioned, Apple has a requirement that applications have a novel component. While it's often questionable as to what is considered "novel" Weather applications get contrasted against the built-in weather app. If the app simply duplicates the functionality it will be rejected.

Are you new to Apple?

haha not at all, it's moreso that it seems like the open source weather app category has a particularly large gap in options on iOS vs Android compared to other categories, which I found strange. But I can see why the usual factors are causing it, as other commenters mentioned.
- Not a dev just a vaguely informed layman. But my understanding would be that the primary API iOS weather apps would use is weatherkit, which is not open source.

Where do these opensource apps get their data from? Usually retrieving weather data costs money when you use an api. I guess those „opensource“ weather apps use an ad model to finance this api calls. May work on a large user base.

Usally iOS users buy their products so i guess gathering a large user base for ad revenue model isnt working for those devs

https://www.weather.gov/documentation/services-web-api

All of the information presented via the API is intended to be open data, free to use for any purpose. As a public service of the United States Government, we do not charge any fees for the usage of this service

https://brightsky.dev/

Bright Sky is free-to-use for all purposes

https://www.dwd.de/EN/service/copyright/copyright_artikel.html

all spatial data and spatial data services available for free access may be used without any restrictions provided that the source is acknowledged

https://open-meteo.com/en/pricing

Open-Meteo APIs are free for non-commercial use

…

Dunno about "usually". There's providers that cost and there's providers that don't. There's certainly multiple free providers available.
Aren't there many govt funded free weather API's around the globe?