I agree if they are fed up about it and their situation, then it is up to them to go out and find something to do then - whether that is a part time job, voluntary work, a hobby, the gym or whatever.
However, since DD started school this month I have definitely seen that it is way harder to work and arrange childcare around school hours, when compared to being able to use a fday nursery.
I doubt very much they all do nothing all day either - think that is very mean to say.
On my days off I do housework, I sort the shopping, I organise household stuff (holidays, days out, visits, etc) but I do also spend some of my time meeting friends for lunch, retail therapy, sitting at home scrapbooking for fun, watching a bit of TV and drinking tea. I imagine SATM do a bit of all that too.
It may also not be financially viable for them to work. Many part time jobs pay peanuts. And term time only, part time jobs can be very hard to come by.
I thinkk you are being VERY judgemental (based on your OP anyway).
I agther you work? Maybe FT? Not envious at all maybe?