For my main desktop I use Mint because it just works, widely supported and Cinnamon is good (sadly no Wayland yet. ;_;). I also use Home-manager for my configuration because it allows me to easily just specify my config as a set of files I can check into git.
For my server, I use NixOS, because having all my configuration in a few text files is very nice to get an overview of what my server is doing.
The mobile and PC gaming markets are very different, both in terms of monetisation and what games people expect to play.
If Valve wanted to get into the mobile games industry they’d basically be starting from scratch, and I don’t think it’s a market they’re particularly interested in.
You’re also assuming that buying a game on PC steam will also give you a license to play that game on android, which isn’t a given. I think many games have completely different monetisation models on mobile vs pc, so sharing between platforms like that wouldn’t make sense.