And you only mentioned Gr.3/GT3 cars. The list is quite a lot bigger once we include everything and from many years, instead of just "up to date".
I personally don't think economy cars should be in the game more than they already are. It takes too much time to model a car, to waste on a car that will be used by a very small niche of people. Instead they should just model their sportier versions (this goes for hot hatchbacks, sporty SUVs, and so on).
As for the GT3 cars, it's always difficult to be "up to date", as those cars get updated/replaced very frequently. Whatever is up to date now, won't be in 1 or 2 years time, so there's really no rush to get every single one of them, other than the glaring omissions (Aston Martin, Ferrari, Bentley, Corvette). I'd rather see them model GT3 cars for their uniqueness compared to their predecessor, rather than just another "Evo" variant of an existing one. I'd rather see the Murcielago GT1 over an Huracan GT3 EVO II for example. Then there are the brands that have no car in the class, like Maserati could use the GranTurismo MC GT3 (or even the MC12 GT1) way before BMW needs their brand new GT3 (BMW has 4 cars in Gr.3). The same goes for Cadillac, with their ATS-V.R.
And once you jump out of the realm of GT3 and race cars in general, to road cars, oh boy the list is huge, from modern cars of various types, to cars from every other decade.
At this point, they don't "need" to, but they do need to up their content releases, as their main competitor has a bigger car list, and it's still dishing out more cars. One would hope that with outsourcing being seemingly a thing now for PD, that they would equal, if not surpass, the rate of cars they woud be adding via update, compared to GT Sport.