Developer that is in his right sense will rather create a veritable landing page or website for people to download his open source software than put it in Google play store. I don't know for Apple store but there aren't much crypto app on their play store because of their strict rules and verification of app submission and also government policy on crypto ruling. A page enough for people to let user download for PC and windows, even Apple can be downloaded on the website, verified with a signature but running apple, you will have to enable developer mode on the settings but of course it comes with a caution.
You do realize that most of the apps are created for profit making not really by a community of people for free?
Building an app takes time and money so the creator should find a way to monetize their products to make profits out of it so even though they manage to create their own platform they can never compete with the two market stores which probably have over billion users so from business point of view they are doing what's best for them.
IOS and android are both smartphone OS and sure we can use simulators to run them on PC but why would people even bother doing it?