Do you have any reason to think that this is more common in shops than in home mechanic things? Because, "left over parts" is a stupid common thing that I have heard from all teardown and rebuild projects. To the point that I'm willing to wager it is more common for home repairs than it is for shop run repairs.
Further, you can always split it. Hire them to do it, but then you inspect it when done. If you are good enough that you can inspect after you did the work, you can inspect after anyone else did, too. Right?
Yes, I think it's more common in shops - especially discount shops. Anecdotally, with my first car, the first quick lube used the wrong wrench too tight on my oil pan bolt and basically stripped it; another friend got the incorrect filter one time.
I may know less, but I'm not being watched for efficiency; I'm less likely to break out an impact gun (partly because it doesn't fit under the car as easily) and I can compare the old to the new. And if I'm tired, I can take a break and check the manual.
And if I do the work, I do the test drive; and if something goes awry, I should know what to look for, for that rattle, that noise, or the next warmup. A quick lube tech might drive your car 30ft to a parking spot, and is likely thinking more about the next car than, any given noise, or whether the thermostat has cycled even once.
On your further, sure you can split the difference, and I do do that when I do take it to a shop, and most of the time I've not been impressed (though I have been with some bmw shops). But this only works if you know what your engine sounds and looks like when clean.
I haven't worked with enough discount shops to really know how well they can work. Biggest complaint I have with most that I have dealt with, is that they don't have the tools/information necessary to work on the modern vehicles, either. Frustrating, as I don't think it is a negative of the shop, necessarily.
That all said, interesting to read your perspective and how different our "priors/expectations" are. Would love to see some collected data to know how well this plays out.
In the vein of "car guys," I'm also curious if it conveys information to see clearly restoration vehicles near local shops. Just this AM, I noticed a classic mustang next to the main shop we use. I know they have fun working on some older vehicles.
Further, you can always split it. Hire them to do it, but then you inspect it when done. If you are good enough that you can inspect after you did the work, you can inspect after anyone else did, too. Right?