Would you rather have it degrade to the functionality of an Xbox 360 controller without Steam? That's the best they could do without all games including support for the controller (latest SDL3 has it).
Yes? What's wrong with that? Then it's at least usable.
Can you hack it together in theory, get something working but making sacrifices left and right? Sure. But why would Valve want to do that? Use experience would be incredibly bad.
But give it time. There will be a standalone driver for Windows eventually, either from Valve or from the community.