Time invested and the belief with better communication/efforts than a fulfilling relationship can result. Lots of people want it and are willing to try for it a few times before throwing in the towel.
Kids. Lots of people have so much of what they feel as love reliant on being a family unit that they’re willing to go back to a partner they don’t necessarily trust or respect in order to have the family lifestyle and optimum access to their kids. Friends and in-law relationships go with this too.
Some people feel that partner isn’t perfect but the closest they’ve had and the fear of not being love any better than that is stronger than the belief someone better suited will come and the willingness to wait out the lonliness to get there.
Face it, when you break up then you break up whatever lifestyle you’ve been living with that person. You no longer have a taken for granted meal partner, snuggle partner, confidant, sex partner, movie buddy, exercise buddy, etc. Maybe you don’t like the feeling of being “Mike” as much as you liked being “Dad” or “Honey”. Maybe you hate the idea of not fitting in with your couples friends anymore, the might shun you for other partner instead, your family might scrutinize you for not hanging in there, not working hard enough.