If a SAHM has young children at home she will have a great deal more caring and housework to do than a full time working mother who has children at nursery or childminders.
Having toddlers at home full time is gruelling. They are like chimpanzees in terms of making a mess, needing entertained ( often messy play) will pull out toys, drop food on the floor, bring in mud, spill juice, pee on the carpet, stick keys down the toilet, smear yogurt on the cat, the walls, the floor, their hair, the TV.
That is 40 hours of caring for/feeding/amusing/cooking for/clearing up after that full time mothers don't have.
If you leave for work in the morning and drop your kids at day care then the house is in the same state you left it in when you left.
Small children at home are very hard work.
Working Mums know this too of course, as they have their children's chaos potentials evenings and weekends.
But being at home all week with little ones can be a circus.