Good point, but I think the worst of it is his refusal to collaborate as someone else mentioned.
He has been so incredibly stubborn that everything be like how he envisions it, he was unwilling to hear any constructive criticism. Even as far as Kingdom of the Crystal Skull, both Spielberg and Ford said that Lucas WOULD NOT go forward with a new Indy film if it wasn't about aliens (inter dimensional beings or whatever). They both had to eventually give in since George was essentially holding the franchise hostage. I'm sure there are countless of brilliant screenwriters out there who would have volunteered to write the Prequels for free if it meant to be involved with the SW universe. But instead he wanted to do it himself. Even with the script polishes Jonathan Hale did for the second and third prequels had done, I'm sure he wasn't allowed to touch certain things like character motivations, etc. Or else Lucas purposely hired Hale knowing he could control him.
Now, I can truly understand that as an artist you would want your singular vision. But George himself has acknowledged that he is not a good writer. Why not give a professional a chance? I can only imagine if George had taken some drawing classes and suddenly he thinks he can do all the production and character design himself!
I think he's always had some poor judgement, but in the beginning he was still inexperience enough, and hadn't become THE George Lucas, that people around him were willing to say, "that's not a good idea", and for George to back down. By the time ROTJ came out, he was much more confident and could therefore push cuddly Ewoks and burping Sarlacc pits. Isn't George's refusal to collaborate one of the reasons Gary Kurtz left after ESB?