But the SE's predated the prequels so I don't see how that world would exist.

Lucas handed over to Kersh because directing the first film gave him a nervous breakdown. He thought he could take a back seat and that would relieve the stress. As it was he was more hands on with episodes five and six than he intended.

I think he directed the prequels because it was simply easier than when he first tackled the films. Advances in CGI made it possible for him to "realise his vision" with much greater ease than in the seventies and eighties - remember, it was seeing Jurassic Park that made Lucas say "now is the time".

I'm sure that ego probably played a part, too. It's his story, his universe, so why wouldn't he want complete and total control of how the films turned out?