Lucas bit off more than he could chew. He had thumbed his nose at the studios and the Director's Guild so was stuck with directing them himself. He always admitted that he wasn't interested in directing actors. His approach with the prequels seems to have been "Stand there, say this and fuck off". Watching the behind the scenes docs he seems more interested in making sure Yoda's ears move a certain way than dealing with actors or rewriting the script. The part where he has Ben Burtt move actors a smidge to the left or right in the edit suite also indicates that the "Stand there, say this" style was deliberate so that he could move the actors around like bits of CGI in editing.
I'm sure he has said he always thought of himself as an editor and that's what he was happiest doing.