I think a lot of people stay together even when they're not in love, and don't even much like their partner half the time, because it is easier than leaving, and they will be better off financially if they stay. Both people in the couple, get used to a good lifestyle, and as long as there is not abuse, most people will stay.
The couples who I have seen split over the years (who had children under 10,) both struggled financially, and it affected the children, and made it hard for them to form relationships. Plus, if your parents are divorced, you're twice as likely to divorce.
Many people, mostly women, (but some men as well,) stay because they're better off staying married/having the OH's extra income. No-one is better off alone! I have seen women who have split from partners, struggling to make ends meet, and scratting like rats to get in as many hours at work a week as possible, sometimes working 50-55 hours a week, just to make ends meet. Not ALL women get big payouts/alimony/childcare payments from their (ex) partner!
And then when the kids have gone, it's too late to leave, because there's nowhere to go, and it's too late to find anyone else. Although most women who have stayed in a loveless marriage for 25-35 years or so, probably wouldn't be arsed with another relationship anyway!