Homeschooled kids aren't taught by just their parent(s). They also learn from the subject matter experts who teach specific skills - I personally was enrolled in piano, martial arts, and swimming lessons, but there's plenty of classes that teach important skills that shore up the weaknesses in the parent's teaching toolkits.

Aside from that, homeschooled kids learn from the world around them. They learn from their own experiences, and they develop their own discipline. They learn how to self-motivate, and how to explore the world around them. It was an eye-opening experience when I went to high school for a few years. I learned so little and wasted so much time, though I tried my hardest by asking questions, contributing to the class, and respecting my teachers. There's just so little that can be done when a single teacher is forced to administer 40+ children.

Meanwhile, before high school I'd sit down and read a textbook for 4 hours (without being told to, just because I was interested). I'd go on long walks and sketch plants that I saw. Some days I'd do quite a lot, and other days I'd be done by noon. Even now I find myself approaching problems in a fundamentally different manner than my peers and I can't help but believe that it's because I grew unimpeded by the early academic process.