What did Lamont do for a living?
I know that Lamont drove around all day finding the crap that they sold in the junk yard via that red truck.
My point is that he seemed to have his act together. He had nice friends and dated some classy girls. Why didn't he get his own place and visit Fred on the weekends to make sure he didn't get himself into trouble with any of his hair-brained schemes?
I guess the immediate answer to that would be that there wouldn't have been much of a show had he moved out. He therefore, never left because that was his home/place of employment. He was his own boss and as Fred would say "heir to all this." Also, because Fred appeared to be incapable of taking care of himself, but in reality, he was just being manipulative.