alt Hacker NewsThe hard part of Linux ports isn't the first 90% (Using the Linux APIs). It's the second 90%.
Platform bugs, build issues, distro differences, implicitly relying on behavior of Windows. It's not just "use Linux API", there's a lot of effort to ship properly. Lots of effort for a tiny user base. There's more users now, but proton is probably a better target than native Linux for games.
It’s not really about OS differences - as the GP said, games don’t typically use a lot of OS features.
What they do tend to really put a strain on is GPU drivers. Many games and engines have workarounds and optimizations for specific vendors, and even driver versions.
If the GPU driver on Linux differs in behavior from the Windows version (and it is very, very difficult to port a driver in a way that doesn’t), those workarounds can become sources of bugs.