Yeah, that's definitely a common reason. But I actually feel that it's one of the worst ones, and that kids would be much better off if the parents calmly explaining their decision to the kids in a positive way, thereby sparing them the pain, conflict, and at the very least unhapiness that staying together would likely bring.
Some kids need two parents more than others. My ex and I stayed together as long as we did in part "for the kids." He physically moved out about a month before our oldest turned 18. I think we did the right thing. I have health problems and two special needs sons. My oldest was quite challenging to raise. I have reason to believe that divorcing sooner would have been catastrophic and potentially deadly for one or more of us.
Since you cannot a/b test this in individual cases and can only say "people who left earlier have these corresponding average experiences or outcomes compared to people who stayed longer," I think it's a terrible disservice to many people with big life challenges to act like they were just being neurotic to stay as they long as they did.
My comment is intended to be taken broadly on average. Based on what you say here, I would agree with you that staying married might have been a better choice in your specific situation.
Anyone thinking about "staying together for their kids" should talk to people whose parents divorced and ask them, "Were you happier before or after your parents split?" Cases vary, but the people I've had that conversation with have all said their parent's divorce was a relief.
I know there are people who claim "divorce as such" is bad for kids but I'm very doubtful about that. Divorce can be good or bad depending on how it's done, and how interested both the parents are in their kid's well-being.
My opinion -- as someone whose parents divorced when I was 5 -- is that merely staying together and being miserable isn't necessarily an improvement on divorcing, but getting into couples therapy and working through your issues is vastly better, for your kids as well as for you, than either a miserable marriage or an acrimonious divorce.