Everyone's circumstances are different. It's easy for those of us in relatively senior positions at work to wonder why men don't just work more flexibly, but I think it's often more difficult if you're further down the organisational hierarchy - you're not in the same position to challenge the status quo or influence how things are done, and you may not even get to set your own priorities/choose how to approach your workload. Senior managers are typically judged by results, not by how late they leave the office, but that's not always true further down the tree, and not everyone gets to decide what their working day looks like.
People respond to stress differently too. Some people can juggle multiple priorities happily and successfully. Others just seem to find life more difficult, and for whatever reason, it may be too much for some to combine FT work and family. We all have different levels of energy and resilience. And perhaps it's easier or more socially acceptable for women to say that they've given up work to look after their children/support their DH's career than it would be to say that they have left their careers in order to preserve their mental health?
And then, children aren't all the same either. Some may have particular needs that make it easier to have a parent at home, while others really thrive in childcare settings. Some get sick more often than others, and more time is needed to care for them. Some families have one child, others have many more, with differing age gaps etc. Some people have a lot of family support, others have none. And so on.
There's no way that I would ever choose to walk away from my career, because I have seen the bitter regrets experienced by my mum as a result of staying at home with us when we were kids. I want to model a happier model of work-life balance for my dd, and with an incredibly flexible job, I am confident that I can do that. However, I know that's my particular frame of reference for this stuff, and that's what motivates me. Others will look at these issues through a completely different lens altogether, and I guess they will reach their own conclusions.
I do regard myself as a feminist, but I don't see those who choose to SAH as letting the side down in any way, or perpetuating the patriarchy - they're just doing whatever feels right for themselves and their families. Are their choices influenced by wider social norms? Yes, probably, but I don't really think that the onus is on individuals to break those norms. And I imagine that we all make choices that are influenced by society in one way or another. Are they putting themselves in a vulnerable position by being financially dependent on another person? Yes, potentially, but they can take steps to minimise their risk.
I'm afraid I do get a bit irritated with the smug assumptions about frazzled working mums who don't have time for play dates or family days out, can't collect their children from school in case of sudden illness and certainly wouldn't have time for any hobbies, as if that's the only alternative to being a SAHP, but I guess it's just ignorance. Or perhaps, in some cases, people feeling the need to justify their choices.
Ultimately, if you and your family are happy with the choices that you've made, then you shouldn't have to justify those choices to anyone.