Why school seems to make sense
A large part of the reason that many parents send their children to school is free childcare.
It helps us understand others if we assume they want the same things we do. In this case we and they want what's best for our children. They're just going about it in a different way. They have legitimate-to-them reasons for believing they're right (and, perhaps, that we're wrong.) How they justify the conclusions they come to gives us insight into why people choose as they do.
Our society has gotten stuck on the notion that the most important thing kids need is an education. Everything else kids need -- socially and emotionally -- is all (supposedly) easily met just by existing in a family and existing in a world where kids come in contact with other people. Those needs, it's felt, don't need conscious attention.
But it seems like a good education is almost impossible to achieve so kids need experts handling it as early as possible. And I think the way the education system is set up, the way society insists that there can be no success without a good education, parents have come to the only conclusion that makes sense: get them in school as early as possible. (And coming from that whole mode of thinking is one of the things that makes getting unschooling so hard.)
Parents don't know what to do with their kids because they're certain what their kids need most is learning. Parents are certain they need to do to help their kids progress rather than be to nurture who they are. They're convinced kids need activities that have a specific purpose, a specific goal and outcome and well defined beginning and ending points.
If unschoolers were to suggest the most important thing is living joyfully with children, I bet most parents would think that's living in la la land since it won't help kids get good jobs and security. Security is a route (supposedly!) to happiness. Happiness is not a route (supposedly) to security.
Maybe part of the problem is that we can't quantify love and friendship. We can see progress if kids do all their worksheets and see lack of progress if kids don't. If we hug kids 10 times a day or just once a day, or they get to play with kids every time they ask or play once a week, there isn't that same sense of progress or lack of progress. There's more a sense of progress if the kids spend time on educational things than there is on emotional and social things.
Maybe a bigger part of the problem is that we depend on quantification to give us feedback to tell us how well things are going rather than relying on our own senses. We've learned to rely on experts to tell us what we are seeing. We've learned that we can't trust what we see, can't trust the conclusions we might come to because they're often different than what the experts say. (And I think this is due hugely in part to school where problems are set up to have only one right answer and the goal is to get to that particular answer. Seeing something different, coming to a different conclusion means you're wrong.)
Lots of things will have to change before parents can see a better route than the educational track.