Some people like to be martyrs. They're comfortable with playing the victim. Staying in an unhappy marriage and blaming it all on their spouse.
Instead of actively trying to improve the marriage, seek marriage counselling and learning to let go, they store it all up and chew it over, until you can cut the atmosphere with a knife. Keeping score of all they've done and feeling they're the innocent party. But they do nothing to change the status quo. Instead it becomes their shroud, their identity. They're the hard done individual who'll stay in the marriage as it's the right thing to do. Is it the right thing? The poisonous atmosphere for the children. The petty snipping. No affection. No love. No touching in front of the children. What model of a marriage is that to set for your kids?
Sacrificing your own happiness to waste years of your life screwing up your kids future as their whole road map of what a relationship is will be based on a lie. But they see it as their duty.
And would you want to date someone like that once their kids grow up and leave home and they finally divorce? If they can hold grudges and unforgiveness so well, then those are not good qualities in a partner. You'd be afraid to ever get it wrong and have it held against you for years to come.
Or maybe life in the marriage isn't as bad as they portray and it's a comfortable medium but to the outside world they like to be a victim? Maybe it's how they get people to date them? Then they can have an affair and stay in the marriage, you're their escape. People like that are damaged. They need to work on their issues. No one can fix someone else. I couldn't, but I thought I could. Only they can make those key decisions. Life is what you make it. Everything you ever wanted is beyond fear someone once said.