I used to port games to linux (from windows). There is just a huge amount of work porting each game, even if the tech used is somewhat cross platform to begin with. Game devs in general (and the people funding them) are just not interested in doing that work; they have more than enough work just getting the game to ship on their primary target platforms. And at least back in my day, native linux ports sold extremely poorly.
Aside from that there is a huge catalog of games using DX 9-12 and other windows-specific APIs that is just never going to get ported and that even valve would never be able to get source code for. Most of that source code is essentially unobtainable and in some cases has passed through multiple IP holders such that whoever owns it now (most likely a big company like EA or microsoft) doesn't even know what they have, let alone how to build it or even where the archives are.
An additional fact is that nowadays many older games have communities that have modded them unofficially by resorting to DLL injection and other hacks - these hacks can work under proton with the windows executable, but would fail on with a native linux binary. An example is oblivion script extender and its many cousins for that family of bethesda games. Many of the more advanced mods for those games require the capabilities of those low level hacks.