Alternatively, it's because of benevolent sexism. Women aren't slobs if they want to kick back and watch day-time TV while watching kids, they're doing the hardest job in the world!
It's not actually the hardest job in the world, by a long stretch. For starters, look at jobs that actively put your life at risk. Giving birth is very risky, too, but once the baby is out I think the risks go way down - however, the riskiness of giving birth is part of why women are given the privilege to get the less strenuous stay at home work and not having to go out as lumberjacks to earn a living. On the downside I suspect the riskiness of birth is also a reason why less women get access to education: if you can only afford to send some of your kids to school, it makes more economic sense to send the kids with the higher survival rate. In some countries one in ten women die from pregnancy.
