What do you think leads mean to cheat?

My ex, whom I’m still friends with, is dating a slight below average looking girl (his words not mine, although I do agree), and usually I wouldn’t call anyone that as it’s none of my business nor am I that shallow however, he cheats on her on a daily basis, with different girls too, he has said it himself, she’s not that good looking but he swears it’s not about looks yet all the girls he cheats with are a lot better looking so why?? Why are you with her if she has nothing to offer except arguments and domestic violence when she catches him cheating?

*she knows he cheats but she will never leave him. They deserve eachother.