Hanging a door is super easy*!

*only pertains to new construction in places where the workers before door installation made sure the rough opening is plumb, level, and square.

Installation in a settled house is often a massive compromise on angle, tilt, placement, and sometimes squareness. When all else fails, bending one or more hinge pins to add friction to the door is the last ditch compromise just to have something kind of work.

I'd say doors are tougher than stairs, at least stairs are stationary. If the floor isn't level where the door is swinging, that bottom gap can be.. Public bathroom worthy.