Controller has a massive convenience factor. Space, setup, reliability, safety. Easy access to keyboard and mouse is a nice thing, you can just play at your desk like normal. People usually already have one for other games anyways.
Quite a lot of modders use controller for these reasons. I know people that use controller because their wheel broke, and even people that use controller who have a wheel already and just don't want to set it up.
Telling people to play on console if they want to use a controller just shows how small of a bubble you use Assetto in. Console games don't have my IRL cars or my local tracks. Try finding Gunsai on console, or LAC, SRP, photogrammetry maps like Santa Monica, which I've turned countless laps with my friends, wheel drivers, for years with using a controller. I also enjoy the challenge of AC physics on a controller as opposed to console games, Gran Turismo feels really bad in comparison.
As to the OP, I know this is an old thread, but there's many different CSP GamepadFX mods available now that makes controller more viable and fun.