OP, I think a lot of the time posters aren’t looking for permission to leave, so much as validation that their perceptions of their relationship are correct, and validation that it’s OK to stop trying (usually single-handedly) to fix a dysfunctional relationship.
From my own perspective, the question of whether or not my H was/is abusive is an important one. My thinking goes something like this. If he is abusive, the chances that he will be able to (or even want to) change enough to save the relationship are slim, and nothing I do or don’t do is likely to make any difference to this. If he isn’t abusive, then he is capable of realising and taking responsibility for his part in the decline of our marriage, and then my actions might influence the outcome.
As to why women don’t just up and leave the minute they realise they’re unhappy, well that’s not difficult to fathom. You build a life with your partner, make decisions about jobs, children, mortgages, childcare, schooling. People rarely say to themselves “I’d better choose option B, not option A, because option B would still work if I split up with my partner and became a single parent, but option A wouldn’t”. Then the realisation dawns over months or years that you don't have the relationship you thought you had, and you are faced with the stark prospect of being unable to continue key aspects of the life you thought you had set up for yourself and your family. All the while, the person who you (once) loved and trusted is telling you that all of this is your fault, that if your life and the lives of your children have to change then that is your doing and your choice, that it doesn’t have to happen if only you would do xyz that your partner wants you to.
You mourn (for want of a better word) the life you thought you had in the past but didn’t, and the life you thought you would have in the future but won’t. All the while trying to work out how to move forward whilst limiting the damage to yourself and your DCs as much as possible. So unless the abuse is severe, it is rarely as simple as “unhappy = leave = happy again”. It can seem more like “unhappy = leave = unhappy for different reasons”. Hardly the stuff of easy decisions.
My H is a glass-half-empty, self-centred workaholic who thinks he should get a medal for the token involvement he has in domestic and family life. Yet he regularly tells me that “where we’re heading” (i.e. separation) is entirely my fault. If it weren’t for MN, I would believe him. By the time most people screw up their courage to post those threads that make your heart sink, OP, they’ve spent moths, if not years, reading Relationships threads and wondering about their own situation. By the time you post, you’re pretty sure that your relationship is dysfunctional and it’s not just you, but you’ve spent so long in the FOG that you can’t trust your own judgement. And the cycle of abuse means that during the “good” patches you second-guess your reaction to the bad bits. So the perspective of MNers outside your situation who can dispassionately comment on patterns of behaviour is invaluable.
I’m sorry those threads make your heart sink, OP. They make me glad – glad that one more person is getting closer to getting out of the FOG and owning their own life again.