I see what you re saying: you re not asking if the cheater has done something wrong (of course they have), your asking why the cheater is the only one who gets blamed for the demise of the relationship when there were probably problems being caused by both parties long before the cheating happened.
Relationships are complicated and nobody is perfect. The person who cheated is not a perfect person (duh), but neither is the person who was cheated on perfect either. However, the person who got cheated on comes out as the better person always, no matter what other crap they did during the relationship. They can drain your savings, withhold sex for years, be a really bad communicator, make empty promises, swear that the next round of marriage counseling will fix things and they ll start working on it after the holidays/summer vacation/the weekend/the work week/their cough subsides, and tell you that you re the one giving up on the marriage when after all this you ask for a divorce and that you should "work on it together" aka I ll keep enjoying my tv show while you adjust your attitude. But after all this, if they get cheated on, they will come out on top. That s just how it is. Take it or leave it.