Are they ported though? I would say thanks to the Steam Deck, Proton is at a point where native Linux ports are unnecessary. It's also a much more stable target to develop against than N+1 Linux distros.
Many are specifically ported to work with Linux without a wrapper, especially among indie games and games from smaller studios.
Unity, Unreal and Godot all support compiling for Linux either by default or with inexpensive or possibly free add-ons. I'm sure many other game engines do as well, and when you're taking a few hours of work at most to add everyone who owns a steam deck or a steam deck clone as a potential customer to your customer base then that is not a tall order.
They do, yet you will hardly find a big name Studio that will waste additional money doing builds, QA and customer support for GNU/Linux, just let Valve do the needful with Proton.
