Steam (via Proton, mostly) already does a lot of development work, and it doesn't care which distro it's on. "Oh no, I updated my kernel and now my graphics drivers don't load at all!" is incredibly common, even with DKMS. Sometimes (mostly) bugs get fixed in Mesa or whatever. Often, new subtle bugs are introduced when a new Wayland/Pipewire/whatever feature goes GA. Having as few moving pieces as possible (by using an LTS distro, or at least something which isn't rolling with upstream and has a modicum of QA) lets you optimize the pieces you need to without worrying that this or that API is going to change underneath you.
Intel and AMD drivers do not have this issue, and Valve was smart enough to not go with NVIDIA, but "I want to be up to date" is a terrible experience.
Additionally, it generally makes for a much less reliable/stable experience (gaming or otherwise) because `pacman -Syu` may at any point break something because you didn't read the release notes, or "mostly" stable features were committed upstream then released so the userbase can put them through their paces and report bugs the developers didn't encounter.
Users of Arch/Fedora Rawhide/whatever accept this, but someone who buys an OEM gaming machine does not need or want this.
Just to note, I AM saying that Arch is unstable. I've been using Linux for 20 years, and I've had my time with Gentoo and Arch. 99% of the tinkering users do is reproducing the work of professional developers at distro vendors who spend a lot of time and effort making sure you never encounter the problems Arch users revel in fixing at all. Sure, you can tell yourself that means you "know" more about the system. But that is time invested that you could have spent doing REAL THINGS, and solving REAL PROBLEMS which are not un-breaking your distro.