Valve basically failed to provide the business value for those studios.
PlayStation and Xbox don't go and cause you constant churn, at least not in the same console generation, and maintenance churn on iOS is only bearable for app developers because there are so many people using it that you can afford to pay the extra effort.
For a studio selling their games via Steam, there is no benefit in making a Linux build.
Their clients still need Steam to run it, and there's no practical different between Steam creating a container with a dedicate Linux userspace or with a dedicated Proton setup.
The audience that REALLY cares whether the game is Linux-native or not is likely the audience that wouldn't want to use Steam.
Actually, Valve could do an Apple and require native ports.
Using Proton to run Windows games is no different of using MAME, UAE,...
I think Valve is trying to leverage the Steam Storefront into a full-fledged Platform. It is not quite there yet.
As such, they have invested a lot of effort into the compatibility layers, which allows gamedevs to support Steam Devices with no extra effort, or minimal effort, which is very important business vise.
As a gamedev, you essentially get a bonus platform for your game without extra dev effort!