> It is so easy that Fortnite (the biggest game at the time, and maybe now) was/is distributed directly by APK.

Amazon Prime Video used to be like this back in the day. You'd have to allow sideloading, install Amazon's custom store, then install the video app from there.

This sort of thing is one of the reasons I like that iPhone is locked down. I know that none of the apps I use will decide they can make reducing my phone's security a condition of using their app. If they want to be in the iPhone store -- and they do -- then they'll have to work with iPhone's strong security.