1. Lazy teaching – parents being unwilling to devote 4-6 hours each day to education. Homeschooling is a commitment that should be treated with the same gravitas as a full-time working position, but many parents have trouble maintaining this mentality over many years of education. This leads to kids that, although they might be bright, just haven't been taught much, which makes them seem dull.
2. Inherited ignorance – if the parents don’t know or care much about history, unbiased politics, mathematics, literature, etc. then the kids won’t either. Obviously, the parent can make a commitment to learn the field at least to a high school level so they can educate their kids, but often this commitment falls through. Because of that, homeschooled kids can lack a multidimensional perspective of the world.
3. Neuroticism – some parents have a tendency to like to make decisions for their children. This can be very dangerous for homeschooled kids, because they don’t learn how to think for themselves or make mistakes.
4. Social gracelessness – a lot of homeschooled kids have very little interaction with their peers. School can provide social acclimatization that parents often don’t know how to give their kids. Especially if the kid is naturally shy, since parents rarely want to put their kids in a social situation where they are uncomfortable. Unfortunately, this can just exacerbate the issue, to the extent that the kids have nervous breakdowns when they finally do have to engage their peers (yes, I've seen this happen).
This is just the tip of the iceberg. I don't want to say that it's impossible for one person (usually, it's just one parent that is home during the day) to provide a well-rounded education to their kids, just that it's hard -- very, very hard. You need to help your kids make friends, and you need to let them be independent. You need to be a committed and capable educator, and you need to give them a dynamic education with diverse experience, not just doing workbooks in the house. Some people are good at this -- they are natural teachers. Other people find it much, much harder.