I think it's just because it was the dominant monetization scheme when they were introduced, people got used to spending nothing up front on their mobile games. Then there are other barriers. Like why would I pay $15 for Stardew Valley when it probably won't work with a controller or output comfortably to a TV. You can do some of that stuff sometimes in mobile, but there's no enforcement of it, so that means you're getting a lesser version of the game, which drives the price down. I wanted to revisit Planescape: Torment on mobile, but they ported it to Android too long ago, and now it just doesn't work with modern Android OSes. They're really teaching me to not treat mobile as a place where people like me should expect to find stuff to play.