Well no one is hiding them and all you need is Credits to get the cars that you want in the game, making every car available from the start is a stupid idea. Gran Turismo will never ditch the credits system and if you don't like the credits system in a game, well go and play something else.No other game comes even CLOSE to offering the same online experience, not even close. I can hop on at any time and generally race in a tight field, that is revolutionary and saves me an astonishing amount of time vs a user lobby experience other games offer.
I just don't see the point in developing all those cars then hiding them all.