I'm sure this has been posted about a million times here already, but it genuinely needs to be a feature and I'm unsure why exactly it isn't implemented yet. There are plenty of games even on Steam which either don't allow rebinding keys/changing your sensitivity or that straight up only support controller. I couldn't possibly know how hard it would be to implement this, but I can't imagine it would be harder than the insanely in-depth controller support was. Steam Input already has a great user friendly UI and the groundwork to make this work great, so I think it's about time it happens.