Cheating partner now ex

Why is it so hard for someone to not cheat, they claim they love you and only wanna be with you but all they did was lie to you're face, I found out my now ex has been sleeping with pretty much everyone, his ex mother to two of his kids another ex which is the mother to another one of his kids, some random people off dating sites and a so called "long time" friend that he claims didnt tell him he wanted to be her boyfriend, all these people so say have partners, one of his exs is married mind they all know I'm with him but why dont they seem to care enough to shut him down or him seem to care enough that he was with someone, I have all the evidence and he still tried to deny everything, took me a while to get the truth about him sleeping with his ex but think he only admitted to that because she told him I knew 😔 hurt and broken is an understatement I'm also 36 weeks pregnant and it's not like we weren't having sex or anything either