Reasons are usually just newest kernel/mesa/etc. Most of the time the difference is very small, and often inconsequential. However, every now and again there is a major development that might make it worth it (IE: The graphics pipeline that all but made dxvk-async obsolete)

SteamOS is arch, so some of the derivatives are too.

Steam shouldn’t really care though.

I think “they prefer” Arch because a lot of them just bought a Steam Deck and that comes with Arch and it just works.

Steam and Lutris work well! I can game on XFCE Mint just fine. I actually have an easier time of it than on a number of distros, thanks to the combination of flatpaks and the Ubuntu base. But, I am not “the kids”.

I’m a complete noob in the games department. Btw, I see that you don’t use Arch.