I would guess it is because doors are large pieces of stamped sheet metal, as are the other body parts they mount to. They span a fairly large opening. There's

I would guess it is because doors are large pieces of stamped sheet metal, as are the other body parts they mount to. They span a fairly large opening. There's probably a certain amount of flex and variation compared to cast and CNC machined engine parts. Doors in a house always need to be shimmed during installation to get the frames square and latches working properly. A car door will have tighter tolerances, but similar issues. The point is these adjustments are (or should be) made in the factory, not with post-delivery reworking.