I've have two theories -
1. Short term accounting which gives design managers incentives to cut a couple of hundred dollars off the "little things" which saves money now but leads to bigger repair bills and shorter lives, thus giving the cars bad reputations and fewer repeat buyers. By then the manager has been promoted and it isn't his problem any more.
2. Too many name brands which leads to artificial quality reductions on the lower end. The Caddy has to be better than the Olds which has to be better than the Buick which has to be better than the Pontiac which has to be better than the Chevy. One way to do this (other than the obvious size and features) is to just build the lower name brands at a lower quality standard than the higher brands. GM doesn't want people buying Chevies because they are just as good as a Cadillac. I'm glad GM is dumping some of the brands so they don't have nearly as many steps down from the Cadillac to the Chevrolet.
I was (and still am somewhat) worried for the Japanese makers starting to separate their luxury brands from their standard brands, although this might be not be done so much because the non-luxury brand is the main company named brand. If they start coming up with lower end brands with a separate nameplate from their parent company, watch the quality on them drop.
Note, neither of these theories have anything to do with unionization. The average union worker is just as willing and able to put on a 10 cent seal as a 1 dollar seal, even if the 10 cent seal will last three years less time. That parts choice is entirely management's decision.
"If they start coming up with lower end brands with a separate nameplate from their parent company, watch the quality on them drop."
Lexus - Toyota - Scion
Scion would be that lower end.