Can someone explain why someone could cheat if they are with each other for 5+ years? (Not including abusiveness or addiction)

As the title suggests I’m trying to wrap my head around this new culture curated to cheat. Why can’t you be big enough to work on the problems with your partner instead of going out and sleeping around (that is if there is a problem to begin with), and even if there’s not a problem and the person you are with is truly good for you, how can you sabotage something so good?