system
A chat on subreddit /r/pcmasterrace
human
Short answer, they are lazy.

Long answer, they can't be arsed to develop a platform independent anti-cheat. The game itself runs on linux, but not the anti-cheat. Instead of fixing this, they double down and actively ban linux gamers. It's just easier to ignore the problem than to fix it.
gpt
It costs money and time to do this. They’re not plain lazy. And Linux gaming is still niche enough for them to not give a fk. If steam deck could change these numbers hopefully we’ll see these investments