First of all, many games can very easily be built and packaged for Linux, devs just don't target it as often because it's a fraction of the market share.

But as for Windows-only games... It used to be because functions games were trying to access simply didn't exist in Linux. Wine is a translation layer that could help with that, but it was both underfunded and had a general focus on all windows apps, not just games.

However these days, thanks in no small part to Valve bankrolling the Proton project -- a gaming-specific branch of Wine that has also contributed plenty of improvements back to Wine itself -- virtually any game you care to play will run on Linux. At this point, if a game doesn't run, it's because the publisher or developer is choosing to not let it run -- likely because of specific anti-cheat software. In the case of Easy Anti-Cheat games like Fortnite and Apex Legends, EAC runs fine on Linux, but the devs chose explicitly to turn off Linux support.