I've asked myself that question on numerous occasions. I'd have to say the main reason is that the sheer amount of replayability in any given Gen keeps me coming back to the games. You basically have hundreds of character classes to choose from, with the ability to customize each individual one (at least to some degree). This alone makes the game worth playing a few times, just to see if you can come up with a better team with a better move pool.
Why on Earth they haven't made an official D20 game yet is beyond me though (seriously, the possibilities are endless).
Rules of fansites:
1 - You can either be right, or you can be happy.
3 - If you think someone is lying about being an expert, they are.
5 - If you think someone is lying about knowing people who work in the industry, they are.
14 - There's no such thing as a source that isn't one-sided.