Because people are so emotionally needy they can't be alone, or can't tolerate themselves as they are. They may have an ideal self image, that is not their real self image. So the needs outweigh the sensibility in needing someone to give them gratification either by respecting them, and / or submitting to them, a person of value. Since the original mate, (in the same situation), probably isn't fulfulling their needs any more, they may be of no value. While their real self can't become it's ideal self, without an ideal mate. Outside recharging is probably sought, as a desire for an ideal partner to compliment the ideal view they hope to have of themselves.
Similiarly, there was a cartoon in Road and Track magazine some time ago. Cars drove through a gate under a sign that said, "You View You". People in desirable cars paid a fee to drive past a long mirror reflecting their image back of what they looked like driving those fancy cars. All for the temporary gratification of seeing what they looked like when related to that particular ego boosting asset. Without it, they would have to deal with themselves, and that is near impossible for a great many people.