I didn't put up with it and split with my ex 2 years ago.
My kids were 5 and 2 then. It was bloody hard work. It still is, being a single parent is hard and exhausting and a lot of people say they don't know how I do it whilst putting up with their shit of a husband. A lot of people know they don't want to give up what they have - lifestyle, house, some freedom, money. They are financially tied, they are reliant on the other for money, security, company, to share parenting duties. And I'm sure part of it is the stigma of being a single parent. They don't want to be alone. They don't want their ex to meet someone else whilst they're left raising the kids, which lets face it, happens a lot.
I can see them divorcing when their kids are teenagers. My best friend's parents divorced when she was 14 and it was horrific for her and her siblings, so I was keen to avoid that - the younger the kids are the better imo, I would rather I take more of the emotional hit than my children. That's not to say it's been easy for them, but my youngest won't even remember us being together.
I have had an early inheritance (my dad died) so I am "fortunate" to be able to buy out my ex out of the house so we don't need to move. Lots of people - most - don't have this option.
As for why I put up with it initially. My dad was a sahd but was also an abusive, lazy sod and my mum worked 50 hour weeks when I was young to provide for us all. I was raised to believe "that's what men are like". My bar was set very low. It was only as I got older, matured, became more of a feminist, saw other peoples' marriages, that I realised that I didn't have to live that life.
I didn't want my children (boys) to grow up believing that's what a healthy relationship looks like, and that's how to treat women. Because I grew up with a very skewed perception of what is a normal, healthy marriage.