The article, 5 Reasons American Women Won't Breastfeed attempts to give some insight into why American moms aren't breastfeeding their babies. I think some of these same reasons might apply to why American moms don't homeschool their kids, or don't continue homeschooling their kids.
1. 100 years of Precedent. Formula has been around a long time. It was touted as a nutritious, modern and "scientific" method of feeding babies and it became accepted that of course the experts could do it better than mom could.
Mandatory institutionalized education is about as old. Before that Americans were quite well educated even with only an 8th grade education. Yet despite studies to the contrary, there is a prevailing attitude in the culture that only professionals can adequately educate a child.
2. Moms are working. Working does indeed make nursing and homeschooling more challenging, but not impossible.
3. Socio-Economic Inequality. The article states that low-income women are less likely to be able to breastfeed their babies because of economic concerns.
In homeschooling I wonder if the socio-economics work the other way - moms who can afford to live in nicer neighborhoods with better schools or who can afford private schools are more tempted to quit homeschooling or are less likely to try to homeschool in the first place.
4. Formula is pushed in hospitals.
In a subtle way, so is institutional schooling. Doctors, nurses, cashier's, the neighbor down the street - everyone will ask the children "Where do you go to school" with the assumption that of course, the kids go to a brick and mortar school.
5. Formula is heavily advertised.
So is school. From the shows the kids might see on t.v., to movies and commercials, homeschooling isn't represented at all, unless it's to show some geeky kid who is "unsocialized."