I've followed the GT series religiously from GT to A-spec, i own a GT4 but i never went deeper into the game. Not that it was bad and i hated it but y'know, it's kinda like waking up in the morning realizing you wanna stop doing something and start anew.
I love cars but i'm not really a car buff, basically..fascination drew me to follow the franchise. Just like you, i like the fact that these games had a vast library of cars to choose from. It's unlike other games that offers a wide selection of cars but with less 'variety'. You can choose to race with those fine-tuned and stickered-down racecars, American muscle cars, classy European cars, Japanese speed demons, trucks, concept cars, and even with mini-trucks that are mainly used for potato delivery (seriously). GT's selection is really on a grand scale in terms of numbers and variation.
Again, just like you, i like GT's attention to detail from upgrading cars to its license tests.
Lame? nah, i don't think so. It's a pretty big franchise loved by gamers and car enthusiasts alike. If it sucked it wouldn't end up having multiple sequels and 'glorified demos' in the first place. Even those so-called glorified demos sell like pancakes. A lot of people love these games.....
GT's difficulty and complex gameplay might probably be the one of the main reasons why some people hate it. It's a 'driving simulator', in order to race and advance in the game you have to pass a series of notorious license tests--merely acquiring the B-level license is already an impossible task to some. Upgrading cars in these games also involves a degree of technicality that may not sit too well with some players. I'm not saying that these people lack sufficient skill to play the game, maybe they're just not interested and willing enough to do tasks other than racing. Reaching the chequered flag first isn't all that matters in GT, it is a DRIVING game.