And old ones wouldn't work on windows...
Honestly I feel like it's better for Devs to have a single endpoint of optimisation, and the proton translates that into proper Linux optimisation.
Most of the times a game has a Linux client I need to manually activate compatibility because it's either not that well supported or the proton version just works better.
At this point, I prefer for Devs to optimise the windows game for proton than to make a Linux version.