Using Steam is even MORE optional. They're PC games! Yet people still want to be on Steam even if you have to play by their rules, because being on there creates value.
Any game developer could at any point opt out of using Steam and accept payments on their own site, or go through another store. Most developers don't bother because Steam provides enough value to make it worthwhile.
It's impossible to determine if this is the case with the App Store since competitors don't (and can't) exist.