It's not one small problem. I can come up with a longer list if you want to. Try flatpak on ARM with Linux. You will see that most apps won't work because they only have x86_64 arch supported in the first place, which is nonsensical since Linux runs on pretty much any kind of hardware these days.
I have been playing on Linux since 2007, so yes, I also know the world of gaming before Proton. And it was much worse, so Proton is clearly a NET positive.