no, it's KY's obsession with detail that takes so long. there weren't many cars added after Prologue, and we're STILL waiting for DLC. a lot of them are rehashes of the GT4 cars and direct ports of GT5P's. throw in the fact that he ADDED details like damage, working windshield/windscreen wipers, parking lights, high beams, WORKING backup lights, full interiors, customizable racing suits, fully articulated characters, day night tansitions, a cutomizable track maker...the man has put uber effort into actually LISTENING to the players, and regularly visits the GTPlanet forums of which Lionus and I are both members (and I've been there since 2005). the only thing he really hasn't taken up is customizable race liveries. I even think I got MY point across, as I kept harping on the point of including info on the cars a la GT2 (which had been removed from GT4).
as for LeMans cars? well, I rarely run them even in GT4. I play with Kei cars.