Linux port if there is one is usually done by a third party porting studio, which is not necessarily at the same quality as the original codebase. Also the devs just don't have the manpower/bandwidth to spare for Linux users given how small this community is.
It's better value for money for both the gamers and the devs if the devs just choose to engage with valve and get their game running perfectly under proton.
These ports are also not usually source ports, so they're not much "more native" than the Proton ports. They often use the same kind of API translation layer, probably also built on WINE. I think as Proton sees more investment and becomes more advanced, it's probably becoming difficult for competing compatibility layers to keep up.
A source port that is optimized as lovingly as its Windows counterpart will probably be faster than the Windows version running via Proton, but the incentives aren't generally there to justify the costs/difficulties. Maybe some day it will be! That would be wonderful.
But until then, Proton seems like an increasingly compelling option for these compatibility layer-based ports of Windows games.
Factorio and Minecraft (Java edition) are two of the few games that come to mind where the Linux port got comparable effort to the Windows port, and I don't think people are in a rush to play either of them in Proton.
Your latest AAA open world RPG on the other hand? Yeah, you're probably going to have better luck in Proton even if it gets a native Linux port.
> and get their game running perfectly under proton
Even better would be to compile for linux, but use DXVK-Native (https://github.com/doitsujin/dxvk#dxvk-native) if you think migrating from DirectX to Vulkan requires too much effort.
They probably QA mostly for windows, so they run into bottlenecks and edge-cases of windows APIs during QA. Linux-native APIs probably have different bottlenecks and edge-cases.
I think the reimplementations of Windows APIs in Linux, even though alternative to the original, should have similar bottlenecks and edge-cases. So the extra QA on Windows helps the Proton version more.
The answer is pretty simple here - hire CodeWeavers to work on supporting your game in Proton/Wine rather than some other porting shop doing an old rewrite-style port.
To be you should compare the windows version on windows, no proton against the Linux version.
DXVK, which proton uses, makes some games run better in windows than "native".
It's better value for money for both the gamers and the devs if the devs just choose to engage with valve and get their game running perfectly under proton.