Why not paint/finish top surfaces that are above the sightline?

It seems to be a standard practice not to paint the very top of doors, door frames, even windowsills if the horizontal surface is up above a person’s line of sight. The same is true for the tops of cabinets where they won’t be stained or otherwise finished.

Controlling costs by reducing effort seems the obvious reason, but I can think of several situations where the extra protection would be worth the minimal additional effort.

So I was curious if, besides costs, if there was some other architectural reason why this is good practice?