Why would a guy want to be with a girl like that?

Ok my boyfriend cheated on me with his ex and then left me for her… He said he does not want to be with her, he just needs to get over her… But i say BULLSHIT. Now my question is, why would a guy want to be with a girl who cheated on him, treated him like shit, they are fighting alot already, and she is a gold digger i believe, she also has no morals! Why would someone what to be with someone like that if he had me, he says im everything he has always wanted in a girl but still he is with her. WTF?