The goal of any cheater is to (hold onto) all that is good in their primary relationship/marriage while addressing their other "needs" on the side. The vast majority of cheaters would rather cheat than go through a divorce. Oftentimes they have 80% of what they want in their marriage but they refuse to do without the other 20%.
They have a variety of reasons why they may step outside of their marriage. ( sexual neglect or dissatisfied, being taken for granted, verbally abused/nagged constantly, lack of romance/passion, grown apart emotionally, not feeling "special", and in some cases simple boredom with life.)
The real question is: Why do we ask why?
Most people could care less about the (reasons) why they were cheated on. The question is a reflex or rhetorical question. The vast majority of people consider cheating to be a "deal breaker". Therefore (why) doesn't matter. Not many of us are seeking "understanding".
The definition of cheating is "breaking the rules". If both people are having sex with others and they know about it and wish to continue being married then what they have is basically a borderline "open marriage". If it's (out in the open and accepted) then it's not cheating in my opinion.