I'm a single parent with a full-time job and the one thing I've realised from my own situation is that haemorrhaging your salary into childcare is not a short-term sacrifice that way many people keep pointing out. If you work a 'typical' 9-5, you will need some form of childcare until your child is 15 (not a legal requirement I know, but this age is based on govt and NSPCC guidelines).
When your child starts school, there are still 14 weeks of school holidays to cover, and unless you are one of the few people who manage to hold down jobs that coincide with school hours, there is also wraparound care to consider. I work 9-5 and overtime is done at home, so I don't work long hours. I was horrified to discover that even once my 2 children started school it was still going to cost me somewhere in the region of £6000 per year in childcare! (I rely on professional childcare as I have no family and my friends all work).
Families lucky enough to be able to call on family or friends for childcare will usually be able to balance jobs and family life more effectively than those relying on paid staff, not just because of financial reasons but also because grandparents will look after a child recovering from D&V for example, whereas a CM or nursery will not. This is why, out of women with children who work, 4 out of 5 of them are doing it with family/friends-based childcare.
I would hazard a guess then that a lot of (unwilling) SAHMs are in that position because they don't have family to help out and professional childcare costs over a period of 15 years are staggering (about £105,000 in my case) regardless of whether they are coming out of the woman's salary alone or the joint family pot. I suspect that if childcare was free or at least tax deductible, there would be a huge leap in women working, particularly among married women who previously wouldn't be eligible for financial help under existing WTC rules.
One of the reasons why childcare costs are considered a woman's responsibility is, IMO, due to the government's treatment of single parents. In most cases, residency usually goes pretty much to one person (nearly always the mother). If that single mother works, childcare has to come out of her income (even if subsidised by WTC). Even if she receives maintenance the cost of childcare is not included in that, so the father has what is, in effect, free full-time childcare while the woman has all the domestic/family responsibility and all the childcare costs. This simply reinforces the notion that childcare (whether professional or carried out by a parent) is a woman's responsibility even among married couples.