Why should I get excited about decent compatibility when I can stick with perfect compatibility? Linux gaming doesn't suck compared to linux gaming 5 years ago. It's still crap compared to Windows.
Try instead coming at the question from the perspective of someone who already has a Linux workstation (e.g. for work) and wants to do as little as possible in order to play a few games—maybe the ones their friends are trying to get them to play as a member of a team. Windows isn’t worth it here: you wouldn’t use it for anything else (so every time you boot into it, you probably have to spend two hours installing updates), and booting into Windows would also prevent you from multitasking to the Linux apps you rely on. Compatibility shims, if decent, are far more interesting to this audience.
Heck, I’d bet money that the Linux casual gaming crowd you described is also heavily outnumbered by people who have a dedicated Windows gaming PC (e.g. me, Mac user otherwise).
Sometimes, in fact, it only takes one or two developers. I can’t think of a good Linux example here, but I know of a good few projects (Dolphin, for example) where the macOS target is supported entirely by the one or two developers on the team who use macOS.