Steam runs games inside of a container to provide a consistent environment for the game. It largely ignores the OS and only uses the kernel as a result. Linux distros have tons of differences that make it otherwise untenable to target. You need an army of package maintainers to maintain per distro packages and sort out differences which seems to work for oss, but doesn't for commercial closed source software.
This is actually a great way of running stuff in "foreign" distros where you can't guarantee the presence or absence of libraries.
For example DaVinci Resolve is packaged for (now quite old) CentOS but runs just fine installed on Ubuntu, with absolutely no problems at all. The only thing that might catch out the unwary is that CentOS packages a couple of libraries as default which Ubuntu doesn't but if you're savvy enough to edit video you're savvy enough to catch this.
You can also run Resolve in a Docker container so it "sees" a CentOS environment no matter what, and it's transparent across any other distro. There's no real need to do this, although it makes running more than one version easy.
Linux is an OS.
It doesn't matter which distro you pick, because they're all the same.