Because schools teach as if every child is stupid. Most kids can race through the material quickly. You can go at your childs pace, instead of the pace of the slowest person in the school classroom. Also, teachers are always repeating lessions. At the beginning of school, some classes take weeks to get into new material, too busy doing "refreshers". You don't have to wait on the slow kids to move forward with home schooling. And you can teach any subject using the child's own interests. Say they like cars. Have them count cars or even types of cars, learn ratios by analyzing their counts, younger kids can learn their colors. Older kids can learn to work on cars, use tools, learn math or even bookkeeping buying parts for cars. It's easier to learn when science, math, reading, research skills are focused on a topic you love learning about. Do this with anything, from archaeology to medicine to sports. "Teaching" doesn't have to be at home or in a classroom; the world is your classroom, the local museum, zoo, bakery, state park, big box store and more.

TwistedTrees said it good. Yes when it's one on one it's amazing how fast they get it. Also helps that you know your child so you know how to talk so they understand. Later on I let them pick what they wanted to learn like computer coding or WW2. If you really look at history curriculum they never really get there too much time spent on ancient stuff that doesn't matter as much for today.. like what type of basket weave.