Why does Leon remain in "Curb Your Enthusiasm"?

Yes they do breakup in Season 7 episode 2 in fact and also this episode shows sorta why Leon stays or is difficult to get rid of to be exact. I would hunt down Episode 2 for further information, I don't want to post the details as they would be spoilers. It sounds like you are halfway through season 7, there is also season 8 for you to enjoy.

Larry tries to break up because he can't deal with the hassle of having to look after Loretta during her cancer treatment even questioning the severity of her condition especially as it starts to infringe on Larry's appointments such as golf appointments. He is then asked to help Leon's friend Alton who is a little down and whose birthday it is and is a big seinfeld fan, Leon also remarks he 'has a fine ass wife'.

Larry gets the idea from a TV talkshow that his personality would not be conducive to Loretta's recovery by essentially being obnoxious and obsessive about small details and suggests to Loretta to meet this doctor. Before the a dinner appointment Larry phones Alton to wish him happy birthday and said that Leon said that he could cheer him up by calling him but then says that he doesn't know why he should be depressed because Leon said he had a beautiful wife. This raises the suspicions of Alton who suspects Leon may be having an affair with his wife.

Larry meets Jeff and Lewis and learns from Jeff that Lewis received a 'blow job' from his new girlfriend. Leon confronts Larry to complain to him that he has been dropped in it by Larry and tells him he has to fix the problem. Then at the meeting with the TV doctor Larry beings deliberately acting up hoping that the doctor will advise Loretta to leave him which she does in private with Loretta. Lorreta then decides she wants to go to a lecture by the Doctor and during the drive to the lecture they recognise the Doctor's husband driving past them and then see the TV doctor appear apparently from the lap of the husband and they deduce that she must have been giving a blowjob to her husband. Loretta is so shocked that she then doesn't want to go to the lecture and now doesn't trust the Doctor's credentials dashing Larry's hopes.

Larry returns home and see Leon rush out with Alton's wife, Leon tells him that Alton is at the front door, we then see Leon help Alton out telling him that his wife is not here. Leon returns to Larry and we see Alton's wife hiding in the passenger footwell and now reappears after being given the all clear, at this point Loretta drives up and she sees the woman appear and it looked like she had just given Larry a blowjob and promptly gives him an earful and leaves with the kids and mother.

Leon appears with soda and chinese food, asks what happened and he tells him 'well I guess this means, you'll be....' indicating that perhaps Leon should also leave. Leon responds 'go upstairs, eat this Chinese food, in my %?*%ing room' and then walks inside, Larry resigns himself to the fact that Leon is not going to voluntarily move out.