Why do people cheat...? 😔

It's just beyond me why people choose to stay in the relationship if they're just cheating on the person they're with. If you're unhappy, unsatisfied or want to "experiment"/see other people, just break up with your SO. I put "people" in the topic because I know that both men and women cheat, but for me, I've only experienced it with men. How do I get out of this relationship? I'm so torn because he has cheated on me before but I forgave him and now I'm almost 99% sure he's doing it again (he's overseas on base) so we are long distance so I don't have solid proof like I did when I visited him but he is behaving the same exact way he did when he was cheating. He told me he'd change because we're having a baby in February and he promised left and right that he would never do anything to hurt me again.... Idk what to do. I'm so tired of being hurt over him. 😢😓