Because lucas signs the checks. Even lucas seems to know it's bad, he just doesn't really give a damn. He doesn't quite understand why Star Wars was iconic by his own admission.
So at this point as someone working for him you have 2 options. Stand on your artistic integrity, tell him it's a steaming pile, and demand changes -- or just take the money and the prestige of working on star wars 1-3. Because you know that the prestige of being involved in the project at all was as much a factor as the salary.
When it's time to throw some shit for the movies being awful, who do we blame as fans? We blame Lucas. I'm sure everyone in "the biz" does too, if they're too polite to say it to his face at a dinner party. But what the hell does lucas care about that? They -ARE- too polite to say it to his face at a dinner party. The shit slides right off him too.
Edit: Hell, JarJar was a retarded character and plot device... but he did LOOK good.
I don't think he ever wanted to make them. Not really. I think he even said as much in the past (pre-making them). Getting badgered into making some movies that he doesn't fully understand the appeal of and didn't want to reprise to being with... well, I guess this is what happens. And it's hard to even blame him personally for not giving a fuck. Not really.