There are a lot if woman on here who are married but effectively are financially single. So they live off their own salary and the DH lives off his. They might share bills etc, but the outcome is that one has more disposable income than the other. Usually the man 
There are loads of threads where married women are even on maternity leave and have saved up for this, in the full knowledge that the DH will not see any need to support them through this time. So he’s sailing out to work as if nothing in his life has changed, while she is trying to buy nappies out of her savings and child benefit.
So if this is the way society is headed (ie letting men have their cake and eat it under the guise of women having “independence”), then it’s hardly surprising that some people simply can’t grasp the fact that in some families, it doesn’t matter who earns the money because it’s all one and the same.
There is a function to a family - ie caring for your children and giving them the best opportunities you can - so parents will organise themselves accordingly, depending on their motivations, personalities and financial circumstances. That’s all there is to it. After a cost/ benefit analysis, it might be beneficial to have both parents working, particularly if the second income makes a huge difference to the quality of a family’s lifestyle; or both would go crazy at home, etc. But equally, two people working might make less sense if, for instance, one earns hundreds of thousands in bonuses and the other is in an average wage because, unless the lowers earner really loves thst job, what is the point if the money is negligible to your family. You might feel that your time is more productively spent with your kids in such circumstances.
If the aim is to financially provide for you family and one person is better placed to do that - then it makes sense for that person to get in with it. If the other aim is to provide a stimulating, loving daily experience for your child and one parent feels he / she is the best person to provide this - then it probably makes sense to get on with it.
Some women have careers that are easier to return to than others. Some women may welcome the “break” in life to reassess what they want to do next. Many SAHMs have no intention of returning to what they did before - they retrain or start their own businesses maybe.
If a SAHM doesn’t back to work longer-term or at all, chances are there is substantial family wealth accrued to enable her to make this decision. Otherwise, she’d be heading back to work wouldn’t she!
So a lot if the “concern” on here is irrelevant. In other words, she knows she could walk away from a marriage in a better financial shape than if she had remained single all those years. It is a conscious choice to live the way she does.
It seems that a certain contingent on MN only wants to acknowledge the downtrodden SAHMs and point out, ad infinitum, the risks. It doesn’t matter how many SAHMs say, “yes thanks, but we’ve got that covered,” they just keep banging on and on God knows why. “Yes, but wait until he leaves you..,, “ “Yes, planned for that thanks....” Yawn.