Simple answer is this - because they want to. Its not about not getting needs met. Their partner isnt getting their needs met either and they arent stepping outside the relationship. Its all about cake. They want the best of both worlds. Cake eaters believe they deserve it regardless of the damage it does to others. They desperately think they need it because they love how important it makes them feel. They believe their life is wasted and they are missing out on the excitement if they dont.
*His wife doesn't give him what he needs because he doesn't give (or understand) what his wife needs *He's insecure and needs to constantly prove to himself he can score *He doesn't love or value his wife. *He's super bored and constantly looking for excitement.
@Caleidoscope quite possible. have not mastered female sex drive to understand the other side but some men can definitely "too horny". Guess that is why some cultures allow polygamy?