I have a different guess after seeing tractors in use as a kid: Driving tractors is a high precision affair. You're moving around expensive equipment that you don't want to bump. You're plowing next to all sorts of hazards you don't want to fall into. You have to park within an inch of attachments before hooking them on.
Good sight lines help with all this.
The difference is in safety margins. You really shouldn't drive a vehicle to within 2cm of any object on public streets. Not even while parking. 0.5m (2ft) is about the closest you should ever drive your car to anything really.
Whereas for a tractor getting within 2cm is normal operating procedure. You won't be able to grab a 500kg (1100lb) attachment and drag it over to your tractor because you stopped too far away.
I've lived in several US states and the law has always been to park less than a foot from the curb.
> park
Do those look like the same word to you? They look like different words to me.
perfect expression of the tragedy of the commons. Until there's top down government intervention, people choose to be either part of the problem or part of the solution.