That particular example is a labor arbitrage issue - it's a better value to have a $500 assembly with $250 labor than a $50 part with $600 labor. If you have the know-how, BYO labor is the magical way to save on this sort of thing.

The worst practices with cars is when they make it too tight or engineered to work on. Ford was kind of notorious for this - some dealers would do cab lifts for routine-ish work to save labor hours. I had a 90s Cadillac that required un-mounting and tipping the engine to change the rear spark plugs, unless you bought a really expensive tool.