Some people stay where they are because the dysfunction has become normalized. When it gets too bad, they simply pursue new and exciting things until the dysfunction becomes manageable again.
Just because a couple goes through rough patches, if they can still get favors out of each other then they will most likely never completely detach. These people grow accustomed to compartmentali zing their relationships. It's like "niam hlob niam yau". They don't have lovey-dovey feelings for the one they have at home, but they can still use her to clean the house. They just need you to be the love-dovey one, but they will never live a life with you either.
Note: A lot of the OGs who marry mos ab from Laos don't completely detach from the first wife unless she is remarried. Many will show all their love and affection to the mos ab, but still have a transactional relationship with niam loj. The niam loj knows this, too. They have no desire to start over so they stay in the marriage just so the mos ab can't have the husband all to herself.