One of the biggest arguments coming from the pro-homeschooling camp is that kids shouldn't have to sit and be quiet for 7 hours a day. It is a little insane that we ask 6 and 7-year-olds to conform to a schedule that mirrors that of a full-time job. While it's highly demanding in concept, it's not like 1st graders are sitting at desks and listening to orations for hours on end. When I was in first grade, I had recess, music class, playtime, and storytime. Sure, I was also learning basic math and writing, but there were a lot of fun activities included.
When you homeschool, you're often losing some of those hours that would've been occupied with some activity at public school. That's not inherently bad, but if your kid is only getting 4 hours of proper schooling a week, they're in trouble. One 9-year-old who couldn't pass 1st grade twice is in the predicament of getting virtually no schooling for her entire life.