Windows kinda has that nice blend of market share (by a healthy margin), performance, and open hardware support (at least until Windows 11 locks that down).
Like, Linux has the benefit of running on just about everything, but its windowing system is probably its most limiting factor. X was originally written to support networked display terminals, and the sheer number of gross hacks it's collected over the years are a testament to that. Wayland, its slated replacement, still has enough outstanding issues that I can't even use it for my work system, let alone a gaming rig.
macOS has performance down pat (no joke, I was using an iMac--running macOS--as my main gaming rig for a while, with Steam in-home streaming filling in the gaps), but there are a few issues there. The first and most obvious one is that they've largely left compatibility behind, what with them phasing out 32-bit and eventually switching away from Intel-compatible processors. This isn't a bad thing by any stretch, considering their $700 Mac Minis now easily rival $2000 PCs in computational power, but it does mean that a lot of titles got left in the dust. The other is that due to Apple's chosen form factor you can't just plonk a 3060 in one of their rigs (unless you're running a Mac Pro) and call it a day. eGPU solutions exist (at least for Intel Macs) but that tends to be an imperfect and expensive solution.
Windows has leading market share, offers solid performance, and allows PCs to have just about any configuration. Furthermore, unlike Linux, devs don't have to worry about users using--for example--a completely different sound system than what they're expecting. It's just one of those cases where the alternatives are just too much of a hassle to support in many cases. Engines like Unity smooth over a lot of that, but even then there are a bunch of weird, random issues. Even simple games like Undertale require additional work and testing to properly support Linux and macOS due to some gaps in GameMaker's OS support (from memory, there are/were some issues when you first enter Alphys's lab on non-Windows builds).
Of course, with Windows 11 they're so eager to copy Apple's aggressive deprecation stance (oh, and they finally ripped off their rounded window borders) that Microsoft basically told people with 5 year old CPUs to go fuck themselves. Considering macOS Big Sur still happily runs on 2013 hardware (and earlier, albeit with minor issues) that seems just a tiny bit absurd. Oh, then there's the fact that the UI is such trash that I legit thought it was a 2010-era Android launcher with a Windows skin on it when I saw screenshots of it.