If someone is unable to bronze the licenses for any given section, they are likely to fail miserably at the races for that level. This person is not going to blame himself for being an incompetent driver, instead they are going to blame to PD for such a (choose your epithet) terri-bad game.
This leads to bad PR, bad word-of-mouth, etc.
If the entire game is unlocked from the beginning for all players, then far too many people will struggle with far too many events, and that is just plain bad marketing. On the other hand, if the group of incompetent drivers cannot even unlock the races, it is unlikely they will complain publicly about GT, because then the gamer is only embarrassing himself, revealing that he cannot even pass the license exam.