So do Jet engine manufacturers, but only after they meet an exceedingly rigid set of quality requirements. The one diff? 99% of American jet engine parts are mfg. in the USA and have a high amount of engine mfg. scrutiny, critical requirements backed up with proof of compliance, and oversight. I doubt than anyone inspects incoming semi-trailer loads of parts coming in daily from us and foreign auto parts manufacturers.
Having spent some time as a quality Engineer at one Tier one auto parts manufacturer and some time as a middle manager at another Tier one supplier, I can assure you that not every part is inspected, but every shipment is spot checked ( say for every 1000 pieces, one gets inspected for compliance, those numbers may vary, but you get the idea).
Parts from "Tier one" suppliers go straight to the OEM assembly lines to be built into new cars, most modern car manufactures have what they call "Just in time" parts delivery (so they don't have to warehouse anything), meaning that parts from those Tier one suppliers generally get built into new cars within a few hours (if not sooner) after arrival at the OEM assembly plant which doesn't really leave them any time to inspect anything there, so all quality control for parts/ components is really on the supplier, GM (or any other OEM really) only does the assembly part of the quality control.
So if GM get crap parts, and then bolt those crap parts together correctly, the quality issue isn't on them and they can shift that blame so their numbers are still good.
Here's one example of number shifting to look good; At the plant where I was a middle manager they would rate jobs on employee ergonomics (repetitive motions, potential for back injury, etc) as assign them a number. Corporate said "make it so no employee at your plant is working a job with a rating of "X" or higher". So, plant management fired all the employees that worked in those jobs with "x" or higher rating and brought in a temp agency to fill those positions, since those were employees of the temp agency and not of the company, all the poor ergonomic positions in the plant disappeared overnight. And the plant management team got a fat bonus for beating the corporate mandated deadline.
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