Because people don’t get married and have children at age 19 anymore in the USA. Not typically anyway.
People graduate either high school or college, have some independence, work, establish themselves a little, get married, save, and then if they are lucky, they can afford to have one parent stay home with their babies if they want to.
Most girls as they are growing up are told to be financially independent from their husbands, so that is their focus initially in adulthood.
Now, that more jobs can be done at home it might change the dynamic. Maybe more women will keep working when they would have taken a break from work 20 years ago.