You do realize that there's a sense of progression involved, right? You're supposed to start from a point where you feel comfortable in terms of difficulty, and work your way up. Some may be more confident and start from Gold level and go from there, or some might start from square one at Steel and go from there.
This isn't simply locking cars behind a gate and waiting for the kind developers to unlock it. If you want it, you have to work for it. How long it takes is up to you. In the case of PGR 3, most of the concept cars, barring the GT-One and the GT90 (personally) weren't even really that great, were middling, or good in specific circumstances that could be achieved with other vehicles. Plus, the open ended nature of the races meant that you didn't have to be locked in to a car class for a race or Arcade chapter like PGR 4.
I really get the sense that you're pissed that nothing is handed to you other then the bare essentials like they are in most modern sim racing games, or thrown at you like an avalanche like Forza 7 or Horizon 4. That's not bad - in fact, it's quite nice to actually have to work for something and to have something to strive for. And the examples that you gave are not anything like Forza 7 (which isn't even true now considering they've now unlocked all the exclusive cars) or Horizon 4.