Asking “why” someone cheated is pretty much a rhetorical question.
Generally speaking no answer they give us is going to cause us to say, “Oh, I completely understand.” We pretty much don’t give a damn about the “reason” or “excuse”.
Like you I have often wondered “why” didn’t they just end the relationship?
Here are some of the reasons often sited.
(Most people are willing to stay in a relationship that is “Not All Bad”.)
For example the mate/spouse takes care of all of the major things one could want however they are boring sexually or their sex drive is lacking.
(I have a friend that married a virgin and she never developed an interest in giving him oral sex. They have been together for over 20 years and he has cheated off and on for much of that time.) Other than the sexual issue they have a great time together, travel, and enjoy a good life.
Not many people are going to file for divorce and go through all of the drama it entails simply because there is of lack of sex, no oral sex, intimacy, romance, passion, or excitement. The whole notion of monogamy and "foresaking all others" is based around the belief that you Have Someone "Commited" to taking care of your physical needs and therefore you don't need to be out and about seeking it from someone else.
It is also difficult for the cheater to breakup/divorce someone his/her family and friends loves. The more ingrained the relationship is in the minds of their "community" of loved ones the more difficult it is to walk away.
The cheating person hopes to be able to (secretly fill that missing gap) while holding onto all that is good in their primary relationship.
It takes “courage” to quit a job without having another one lined up.
Unfortunately this is also how a lot of people are when it comes to relationships.
They aren’t willing to give up a “known” for an “uncertain future”.
It’s very common for a person to have someone “in mind” or on the horizon to potentially date before ending a relationship.
The first lie in cheating is the one the cheater tells him/herself.
It’s the lie of “justification” which gives them permission to move forward.
No one who feels “justified” doing anything is going to feel “guilty”.
This explains why most cheaters blame the person they cheated on for their actions.
In all honesty there are a variety of reasons and circumstances as to why one cheats.
Sometimes it just comes down to being bored, an unusual opportunity to be with someone “out of their league”, an ex comes to town and stirs up old feelings, or as often is the case someone “stopped” doing the things that caused their mate to fall in love.
Again if you are the person who was betrayed or hurt then there is (no answer) that will make it alright in your eyes so why bother asking “why”?