Harry, I don't believe the manufacturers set out to make mediocre kits at all.
No, they don't say "hey, let's make a so-so model of a XXXX."
But the fact is, they know that "good enough" really is good enough for most of their customers... and so the level of refinement and accuracy and detail is generally lower than the typical military kit, whose buyers do demand accuracy and intricate detail. And are willing to pay for it!
Sure, you can cite any number of specific "good" car kits over the years, but overall, the level of refinement in model cars as a whole isn't at the same standard as that for military/armor models, basically because it doesn't have to be... because the customer doesn't demand it.
I'm not saying that the car kit manufacturers intentionally make so-so kits because they're some sort of evil cabal of Snidely Whiplash types who are looking to screw their customers. I'm saying they make so-so kits because they don't have to do any better... their customers are willing to settle for less. And they know it. It's not their fault. It's our fault!
And to your point about so-so models that sell out of some sense of nostalgia... I get that. If you readily buy a bad kit because it let's you relive your youth or whatever, that's fine. Your money, your choice. But what I'm saying is that the manufacturers don't care why a particular kit sells so long as it sells. Period. Your personal reasons for buying a kit are irrelevant. The bottom line is, old re-releases continue to sell, for whatever reason... and as long as we keep buying them, we'll keep getting them. If a manufacturer can keep reissuing an old kit in a new box, they will do it forever. As long as it sells, and makes them $$$, they'll keep on reissuing and reissuing and reissuing...