Originally cars were laquered by hand with a brush before spray processes were invented and became common- it just takes a lot of labor in sanding and polishing. Spray is a time saver.
If you have enough time you could paint a car with a roller. It all depends on what your time is worth, I guess. Mine is worth more than that, to me anyway.
You can also hand excavate a basement with a shovel and pour a concrete driveway with a wheel barrow and sackrete but you trade labor for material cost.
