Not just un-owned cars, un-driven cars... Who doesn't already have hundreds of cars in their garage that they've never driven? What's adding a couple more cars going to do for me? It's like ∞ +3 = ∞
And in all honesty, I think the FH physics are a bit overhyped in the sense that cars tend to all feel pretty darn the same, especially with upgrades. If upgrades weren't a thing, and instead driving stock cars was a thing, then more cars would be more significant, as each car would add a unique driving experience. But with the S2, S1, A, B C classes, it's almost guaranteed that if you need to drive a car for an event, you'll swap out the drivetrain for AWD, suspension, transmission, upgrade this or that in the engine, reduce weight, etc., and woila, mostly now feels the same as every other car you own, especially after you add some wings and downforce..
Sure, in some instances the difference in PI doesn't allow many changes, but overall, once you've applied a couple of upgrades, I'm 99% sure that if you replaced the car model with a blank model and asked someone to guess what car is being driven just by the driving physics, there's not enough uniqueness or character to differentiate cars.
BUT, FH does great at somehow hiding it, I mean with the car models, the sounds, the driving physics, etc., cars do feel like you would expect them to feel; if you get in a 1980s Camaro, it does feel like driving an old heavy slow Camaro that doesn't want to turn. Same for modern/old BMW, Ferrari, etc. or even modern super/hyper cars. Not that I've driven any of those cars, but like I would expect, for that it's fantastic. But once upgraded to say class A, Camaro won't feel that different from other class A cars; maybe its specs won't allow certain characteristics, but if you went blank model, asked what car is that, or even is that a BMW, Miata, a Ford Focus, 1980 Camaro or a Truck? You probably would have trouble guessing just by driving characteristics..