Who knows...but its a pretty good question. I know for JEDI he practically did direct it. Since they are his films he is everywhere and always has an opinion so he figures he might as well do it all.
He darned near had a heart attack from working on ANH, hence the reason why he had Kirshner and Marquand direct the last two. If you watch the bonus DVD, Lucas started to have chest pains 3/4 of the way through making ANH. He was diagnosed with exhaustion and hypertension, not to mention all the problems with ILM and the British crew. Once he got out of the director's chair, things were a little bit easier to control... until ROTJ. Marquand is a good character director, but is horrible with special effects, which is when GL stepped in again.
And the sad part during the making of ROTJ, he and his wife were divorcing.
I've heard that as a director Lucas has trouble working with actors.
__________________ "I realised that there was this incredibly benevolent force that wanted me to know that there was no reason to be afraid... ever." "We don't have to win, we just have to fight."
"There is no way to peace, peace is the way." "Be yourself. Those who mind don't matter, and those who matter don't mind."
considering the crew gave him a hard time, being his first job AND the actors not believing in the movie, I can find myself in him having troubles with them yes
I heard that he suffered from hyper-tension and exhaustion on the previous Star Wars film. So he wasn't too keen. Also he was finacing the film with his own money, and trying to set up a bigger scale production company, he couldn't do that plus be directing on the set everyday.
From what I've read he did a fair bit of directing on Return of the Jedi. Some people say Richard Marquand was fired and George took over. Other things I've read say he had to be on the set to help because Marquand didn't know anything about special effects. I've also heard he directed all the scenes between Luke, Vader, and the Emporer.
I think it's good that he did hire other directors and screenwriters. Lord of the Rings had the same writers and director, and it got rather dull and repetitive in a lot of places. By getting other people Lucas ensured that every film in the trilogy was different in pace and tone and would be a unique cinematic experience.