Probably because they’re big meanies.

The market share thing is real though. Computing is like 1% actually making hardware and software and 99% about getting the humans to agree with each other about how the hardware and software is meant to work, how pieces cooperate, and what it is meant to mean when any given piece does any given thing.

Proton et al. are amazing, but swapping out the whole system underneath a program for one it was never tested on, to provide APIs that are not actually expected to vary in their implementation details, and using GPU drivers that weren’t extensively tested by the manufacturer in exactly these circumstances and individually tweaked to do specific things for that specific workload, is necessarily going to get you a worse result than doing it the way the program authors expected.

And you don’t need very precise numbers to know that Linux is much less used on the desktop.

Maybe with developers targeting and testing on Steam Deck the situation will change, but trying to get two things to work together when only one of them is willing to change for it is extremely hard and I understand why one might compromise principles to avoid having to do it.