I'm not a racing fan, so I don't really care that much. But my friends are all about Forza and Gran Turismo, mostly for the customization and tuning. I think they spend more time tweaking their cars than they do racing.
Too much simulation. Even though they're great racing games, they tend to be a bit bland and boring unless you're really into cars and setups. I like cars a lot, but not enough to spend hours and hours of testing to shave off a couple thousands of a second on my lap times to be that much faster than everybody else. In real life, that'll be cool, in a game though it feels too much like a waste of time. Racing games with the best balance of simulation and arcade style are usually the best which is why I can't wait for GRID's long overdue sequel since the first one was a perfect balance of those two.