Because people treat apple as if it's the pinnacle of design, performance, and usability. People who want to game on their hardware should have it so hard that they will take anything they can to make that possible on that hardware. If Linux becomes that thing that makes it all possible, so much so that it becomes common knowledge "if you want to game on a mac, install linux", then finally some fruit worshippers could possibly see what surrounds them instead of their myopic view of their fruit god.
Mac missed the gaming train two decades ago.
It's never too late for a corporation of their size to wake up one day and decide "well, now there's some kind of a reason to make gaming on mac possible". I can only hope that if they're pushed to that, that linux is so far ahead that it becomes difficult to get back their followers. If by some miracle Valve decides to invest in making linux work on mac, there would be two giants in the game.