Most Helpful Guy
Because then they have to face the consequences of their actions, not the obvious ones i. e. divorce/break up but rather the fact that what they did was selfish and wrong and they commited the ultimate betrayel. In every ones mind they are the hero of there own story, by acknowledging that they cheated and no one but themselves are to blame you take away that illusion that they are the good guy/girl that they are the hero and show them as they are. Many people can't handle that.(currently responding to a woman who is doing the exact same thing (cheated on her husband only a year into there marriage and she is trying to justify it). People can be really shitty.
Most Helpful Girl
They can't handle admitting that they are a shitty person who made a shitty decision and try to blame it on other factors, to make it seem like they "didn't have a choice", when in fact they had multiple.