80% is really impressive, but another way to state it is that 20% of the games you try won't work. I don't think I'd put up with that if I was a gamer. I guess maybe it's not evenly distributed. Do a higher percentage of the most popular games work?

That 20% is mostly covered by competitive online multiplayer games that use kernel-level anti-cheat systems which will only work on Windows. There's not a whole lot Valve can do about that, other than continuing to push Linux for gaming and hope that it gets popular enough to create an incentive for anti-cheat providers to start targeting Linux as well.

I think it's largely to do with the whether the games are PvP multiplayer or not. I.e. many such games have anti-cheat systems that embed in the Windows kernel (or something like that - my Windows internals knowledge is... slim).

I assert that most people who're happy running Linux on their desktop (for games or productivity or development) do not overlap much with the people who're happy to take kernel patches from UbiFuckingSoft. And this includes those people who're willing to take closed-source NVIDIA drivers.

Games with kernel-mode anti-cheat consistently don't work and probably never will (barring them having it removed or made optional). Titles released more recently are more likely to not work simply due to not having had fixes applied to them, although a rather large amount of newer games work fine out of the box if they aren't doing weird stuff. Other than that it's a toss-up, since while it's usually the same few things that prevent games from working properly on Linux, it's not something you as Jonothan S. Gamer will know about unless you go and do research and check ProtonDB and whatnot.

A good rule of thumb is that single player games generally just™ work and that older games generally just™ work.