I think it is was a strategy to make the game longer because of its lack of content(A-Spec events). Also, I think that´s why you only win prize cars once. I think it sucks because there has been days I got bored really quick playng GT5(grinding), and this is something that should happen after at least a year of playing it.
@mykem There was an easier one which I personally loved. Beginer on Costa d´Amalfi. 2 races, 2 laps each, a Rally Concept Toyota worth 265,000 Cr. I did that like a million times.