Because there is no need. It just the usual trend/hype.

  * Manjaro is Arch.
  * Cachy is a patched Arch (exactly what Arch avoids, heavy patching). 
  * SteamOS is Arch. 
  * Arch is Arch. 

> When the patch works it will finally land even in Debian Stable.

Which is very pointless if it's three years late for e.g. a game release

> Any useful and stable patch will be merged by upstream. That is why using CachyOS or ClearLinux isn’t beneficial in long term.

Seems like you're blinded by your own context, if CachyOS for example see patches, integrate them earlier than upstream, and let user use them today rather than "long term", how is that not useful or beneficial to the users who want/needs that?

Besides, testing patches this way sounds like it'll have wider impact in the community than just the distro that integrated the patch, as it'll have a way wider testing userbase then. Isn't that also good long term?

It's about sane default packages and installers and desktop experience, as well as onboarding.

It doesn't take a lot of work to get any distro to become a good gaming machine, but it does take some work to make it a seamless turnkey gaming machine for the masses.