Because can you imagine the outrage if GTA6 or the latest Call of Duty is called “an unsupported game”?
I can see the youtube titles now: “SHOTS FIRED!!! Gabe Newell attacks Ubisoft by warning gamers about buggy games” with a nice video of someone launching the latest tom clancy wankfest and showing the pop-up.
Same with all the fun “quirks” that make sites like protondb so useful. You don’t want someone to accidentally play Dragon’s Dgoma with no cutscene audio or Kingdoms of Amalur with busted ass shaders.