Imagine this:
You pay a builder to build you a house, he does a lovely job and you are very happy with it - the roof doesn't leak, the central heating works just fine, and he's finished it all off with a coat of magnolia paint throughout. He builds the whole house in 2 months, on time and within budget so your are happy.
You then move in, and decide to decorate one of the rooms - you spend 2 weeks doing just that room, and the finished result is fabulous - wonderful colours, beautiful finishing, no drips.
It's just the same with GP4 - no-one could convincingly argue the graphics aren't excellent out of the box, but there's much much to the game than car paint jobs. It's no surprise to me that if someone spends weeks or months working on those alone they can do a better job, but its just not going to be viable for the developers to spend so much time on it.
Ali