Pregnancy and birth are work, and most importantly, only women can do it, and they can only do it once every 9 months. It is a skill that is highly in demand (male sperm not so much), so a woman could potentially get a lot more in exchange for it than "nothing".

One thing women usually get in exchange is the privilege to choose to stay home with their children.

Now how hard it is to stay home and provide for the children every couple has to decide for themselves. But yeah, I personally will claim that it is not as hard as most jobs, putting a roof above the families head and so on.

You can also check the cost of household services (cleaning, cooking, washing) to estimate the value a male housekeeper brings to the table. I think a husband as housekeeper is usually more expensive.