Interesting one. I have been a SAHM when my kids were under school age, and also worked full time with 1 in school and 1 under school age. That was very, very tough and if we had a third child I'd never want to do it again if I didn't have to.
My two are now in school/school nursery 5 days a week for standard school hours. I now work 3 days a week, but for a short while, I had no job, and whilst it was lovely, and my house had never been so clean or tidy, I did find myself thinking what on earth do SAHMs of school age children do every day?! I also felt huge guilt that I basically had all this time where I could be doing something to contribute financially, but I wasn't. Sure, there's housework to do, but I really don't see how you can fill 6-6.5 hrs a day, 5 days a week with 'essential' chores if you have no children at home? Obviously some have the cash to shop/gym/coffees/go for lunch etc etc, and that's fine, but we were not in that position. What annoys me is when such women (SAHMs of school age kids with plenty of money for stuff as listed above moan about their lack of time to 'get things done.' I also think it does become a bit of a self-fulfilling prophecy- I have lots of friends that are SAHMS with school age kids who say this, yet have cleaners, no job, no kids at home during school hours etc. I think it's entirely possible to convince yourself you are so busy and couldn't possibly work.
What I will say though, is I do sympathize with people who say it's hard to find a job that fits in completely with school hours. There are not many jobs that start at say 10am and finish at 3pm, and I guess for many people, the salary they'd earn is not really worth it once you've factored in before and after school childcare, so they choose to continue being a SAHM even when their kids are school age. I am lucky in that I love my job, and it does fit around school hours for the most part.