Part of it is that Steam seems to have a lot more features than those app stores, especially user facing features. If you’ve ever browsed around Steam’s UI and store, there’s a LOT of stuff going on.

The other part is that you have many actually useful alternative avenues to selling PC games. Yes, Steam is far and away #1, but lots of devs go direct to consumer themselves, and some use alternate stores like Epic, GOG, or Itch.