You could also think that, because of WINE, Windows is first class and then they check Linux with Wine.
Depends on the perspective.
PS. Yes, steam detects Wine
By making Linux compatible with Windows games it gets rid of that objection "I'd move to Linux if it weren't for my games" which was the remaining objection for a LOT of people.
Because Steam tracks WINE that's a very good thing, so they can detect players who bought their games to play it on Linux.
This helps encourage Linux native gaming growth, because developers can see the chunk of Linux players rising as more get rid of Windows because they no longer have that remaining obstacle.
So, yeah... maybe that means most devs will just say "my Linux support is just it runs on Proton probably, good luck!" but the thing is... there are games on Linux now. Lots of them. Lots of good ones. I was playing The Witness last night by just pushing play on Steam. No winecfg or winetools or separate DriveC Steam installation. No messing with drivers. I pressed play, the game loaded, I played it. I've repeated this loop in the last few months with most of the games in my library. Endless Legend is back in my rotation again. All of the dumb anime Japanese games where they don't even know Linux is a thing that exists suddenly work. It's glorious.
Wine/Proton may be the lazy way to develop for Linux and might not give people the coveted title of Linux exclusive gamer, but it's working really well if all you care about is playing games and not installing Windows.
Also the "no tux no bucks" philosophy many Linux users take in avoiding paying for non-native games.
Contrary to FOSS folks, professional game studios aren't religious about APIs, as long as there is money to be made.
Whilst this is cool I think it may also, sadly, be commercially irrelevant. Why bother worrying about Linux compatibility when only a tiny (i.e., somewhere between 0% and 1%) number of players/purchasers will run your game on Linux?
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.
Put me in the skeptical camp, I've been hearing the same line with wine support for nearly 2 decades and never found it to be remotely true.
I've had enough trouble getting the actual linux supported games to work. Some work with gnome+wayland, others only work in gnome+xorg, some silently fail, some just freeze, open source AMD drivers still crash the whole machine, etc.
That is especially true if you are forced to use professionally maintained Linux without root access.
You almost convinced me so I checked out ProtonDB, and found out you're overstating things.
Only 50% of games are rated "gold+", and most of those are native Linux versions. Most Windows-only games have issues.
I've been playing Skyrim, Witcher 3, Dark Souls 3, Castle Crashers, Overwatch, Heroes of the Storm, etc all of which are Windows-only but now WINE is so good it runs like it's native.
Linux gaming doesn't suck anymore and it's coming to eat Windows lunch. Check out ProtonDB to see compatibility with your favourite games and the Linux gaming subreddit for more news.