People cheat for a variety of reasons.

Maybe they are lacking intimacy in their relationship and looking for an escape
Maybe they are just horny and can't keep their pants on
Maybe they are just bored and looking for a thrill
Maybe just because they can
etc. etc.

Ultimately, people cheat because of selfish reasons, even if it could be justified. If the base value of the action is selfishness, why would they behave in a way that is moral and selfless? Would it be logical and rational to conclude that a selfish person will break up first before getting their rocks off? No, they'll maintain the benefit of the relationship while getting extra from the affair.