There is a bit of a rose tinted view of staying in a relationship which is desperately unhappy to 'keep the family together' or of being a single parent (particularly without much of a support network).
People do not generally (not saying never but generally) end healthy relationships where they have children. People leave when they (and often their children) are mistreated and often to protect their children from awful environments.
I was in a financially, emotionally (and occasionally physically) abusive marriage. I hid it all from my children. They would tell you they were happier when their dad and I were together, despite the fact that they they really dislike spending time with him on his own. The truth is I spent my whole life organising them so they didn't trigger their dad and have him kick off. No friends over (theirs or mine). Never making what anyone else likes for tea. Never putting what anyone else wants on the TV. They don't remember all of that. Taking them for a 'surprise sleepover at nana's' when he came back from the pub, vomited and shat everywhere and passed out in the bathroom so nobody could use it. As they have got older (late teens) I am more open with them about why things ended.
When I was a single parent of 3, with no village to speak of (parents elderly and ill), none of my children ever got to do things on a 1 to 1 basis. If I was sick/ down there was noone to hold the fort and prevent that from effecting them (despite the superhuman effort that single parents have to put in when these things happen).
I met my DP 3 years after I split from their dad. We dated for a year before I introduced him to the kids. It was another 3 years before we moved in together as we wouldn't do that until we could move to a house with plenty of space for all the kids.
He is a loving supportive parent figure to them and me to my SC. Before we moved in together we talked about potential challenges and how we would deal with things.
There have been times when there have been various tensions between the children, in much the same way as I did in my bio family as a child and teen.
We set an example of a mutually supportive, loving and considerate relationship that my children would never otherwise have had.
Is the current blended family situation perfect? No. But no less so than the alternatives.
And it's fucking shitty for people in happy marriages to act as though people who get divorced do so on a whim with no thought for their kids and that all step parents are selfish individuals only interested in shagging their partner (not an insult ever leveled at married couples, although I see plenty of parents prioritising their spouse over their joint children).
It's just life, and people mostly trying their best.