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).
A Winner Is You!!!
If you spend too much time thinking about a thing, you'll never get it done.
- Bruce Lee
It is amazing that people who think we cannot afford to pay for doctors, hospitals, and medication somehow think that we can afford to pay for doctors, hospitals, medication and a government bureaucracy to administer it.
- Dr. Thomas Sowell