Because it kinda is negative to resuse models to have such a huge quality gap on newer hardware compared to PS3's car models. It's hardly the same between the PS4 & PS5 when that gap is so much smaller.
When I said "better model consistency", I'm talking about the fact GT Sport right now at the very least doesn't use PS2 assets, leaving more consistent visuals across the board. Gran Turismo 6 (as much as I liked it), didn't have that. Oh for sure, I used some of those standards, but I would be lying to myself if I thought having them there was a good idea.
Also, while you having a point about GT Sport having some of its own issues with duplicates. They still are nothing in comparison to the 30 different NA Miatas, mult-regioned variants of Mitsubishi 3000GTs, multiple variations of R34 Skyline GT-Rs, or mutiple liveries of Super GT cars counting as different cars in GT6.
When there's 800 car models in a racing game that look like this;
While the other cars look like this:
Then there's a problem.