> Targeting Linux means probably targeting all distros
Valve actually distributes a runtime (or at least used to), that's based on Ubuntu, and provides a stable target for developers who want to release a Linux port.
But I agree in general with your point; if the Windows build already performs great on Linux through Proton, why go through the effort to release a native Linux build?
They still distribute runtime(s). Proton runs inside one of those runtimes. You're talking about 1.0 version, 2.0 was based on debian 10, and 3.0 is based on debian 11.
It still has some assumptions about host system, though, but that's a problem for those who package steam. For example, my non-FHS NixOS provides everything required, and it works out-of-the-box.