I've heard this before. So as a question. If I was to repaint my truck to stock paint but all the steps needed for a good quality paint job. Would it still not come out like the original paint when it was brand new??
Painting can be subjective. A lot depends on the quality of the paint and how thorough the underlying prep work is done. Still on a high end paint job the paint prep is water sanding with blocks etc. Bodywork the same way hand sanding with blocks. The machine sanders only get you so far so it a very tedious labor intensive process.
At the factory, which I have been to a few, the cars are painted with nothing in them and the paint is baked at close to 300 degrees on a repaint we have interiors wiring in them so baking is usually limited to around 140 degrees. It's debated if the baking temp makes a difference in durability with a repaint as to whether it will be as durable as oem. I don't really have an opinion on it as I have painted cars with higher end clears and even some mid grade clears that looked good going on 10 years sitting outside and it's hard to get that out of a factory paint job.
Our trucks, the late 80's early 90's some were painted with lacquer basecoats and enamel clears and some were painted single stage. Gm had some issues as we all know. Urethanes were new to the market and in their infancy as far as development so todays materials would be better in that regard.