People cheat because they want to cheat. Cheating means lying to, controlling, manipulating, exploiting and conning a steady partner. People who do that often know that controlling someone through lying at that massive level and to that massive extent is wrong so they invent excuses, which are often based on claiming it;s the cheated on partner’s fault. Is that any surprise? I mean if you can lie to that extent, what keeps you from lying to yourself?