Because it's intractable on Linux and advocates don't want to admit that. The entire security model on Linux is resistant to deeper levels of access and control for applications, which is required for kernel level anti-cheat. While these forms of anti-cheats can't stop cheating, they are clearly more effective than user-space anti-cheats. For 99% of users, we gladly accept these more "invasive" anti-cheats because it means less cheating in the games we enjoy. Linux developers will never allow this kind of access because it is antithetical to their ideological beliefs around security. They gladly exclude any kernel level cheats to maintain the security model. It is a permanent impasse. One which I believe will never be solved with user-space or server-side detection. This is why the most common retort is: "just play different games."
To be frank, the argument that kernel level anti-cheats are invasive has never been all that accurate or compelling. Any user-space application already has numerous privileges which could ruin your day. You trust a developer and application every time you run it, irrespective of its access level. Valve has an opportunity now with SteamOS to impose technologies like SecureBoot and "safe" deeper layer anti-cheats which actually work. Yes, Linux enthusiasts would be up in arms, but it would mean that the most popular online FPS games would be supported on Linux, and I think that's far more important.
Well, it's not intractable if it's pushed to the underlying hardware and signed drivers.
Valve could build something into their chipset and start signing the Steam Deck drivers, create secure boot etc and essentially create an Apple SIP equivalent. Wouldn't work for the rest of the Linux ecosystem or other devices, and people would absolutely howl about it, but they could do it.
The other side is linux totally permits you to do whatever you like to your system, and then it's similar discussion to DRM (digital rights management, not direct rendering manager). When you're trying to the user from doing things they're not allowed to and the same user can fiddle with the system, there's no starting point for trust.
To be frank, the argument that kernel level anti-cheats are invasive has never been all that accurate or compelling. Any user-space application already has numerous privileges which could ruin your day. You trust a developer and application every time you run it, irrespective of its access level. Valve has an opportunity now with SteamOS to impose technologies like SecureBoot and "safe" deeper layer anti-cheats which actually work. Yes, Linux enthusiasts would be up in arms, but it would mean that the most popular online FPS games would be supported on Linux, and I think that's far more important.