I would never want an open relationship. If my boyfriend cheated, things are over, no questions asked. If he asks me if I'd be open to an open relationship because he wants to fuck some other girl, it's also over.
Cheating is in no way the same as an open relationship, though. With an open relationship, nothing is being hidden and both partners will be aware of all the sex/other partners. With cheating, it's a major betrayal of trust and respect for your partner/relationship.
Honestly I think it's because people know there are few people (men and women alike) who would embrace the idea of an open relationship or a polygamous relationship. So they cheat to satisfy that need in secret because they don't want to lose the benefits of the relationship but they also get to have the benefits of not being faithful.
Since most people know the chances of finding someone cool with that is kind of low. Plus they won't get as much of a rush with sleeping with other people if the other person doesn't care. When people cheat they get a rush since they are doing something that they shouldn't. Along with the chances of getting caught.
Because people don't know what they truly want. They battle between fantasy and reality. People have this great thing for pretending to be a good loyal person when they're really just leading others on.