I think that the problem is that we don't have enough OPTIONS to keep our interest.
Think about it this way: I like racing, but, honestly, I'm getting bored of it.
If I could self-generate races (even with some general buttons like "easy, med, hard challenge", then "short, med, long races", then "easy med hard tracks" I would be FAR more interested in seeing what races could come up. I wish that the seasonals and quick match could have even two tracks and two lengths to be joined to per level.....
The other thing is, I don't ALWAYS want to race. Sometimes, I would like to create something. So give me a livery editor and/or track generator. I could spend QUITE a lot of time building a track. Maybe some time building a livery. I like the idea of photo mode, but staging shots aren't my forte. (I rather stink at it....)
So, it's not all about credits. It's about the overall experience.
And there aren't enough options to keep me seriously interested or coming back. Mind you, I race every day. But, I wish I could do something else when I log in, as well.