> Second because of the hubris: what makes you think that you are actually more qualified to teach your children than a professional? In your case it may very well be true, but I fear that people too dumb to know better will also think they are qualified to teach their children, and ruin them in the process.
There are two reasons (and both have obvious analogies in the software world, so we're almost back on topic here ;)):
1. Because I went through university alongside a group of people on the "teaching track". This is what we called them because their stated goals were to become public school teachers, but it became a slur in the rest of the department, because not -one- of these people understood the material she (or he) was studying. They all worked hard to find 120 credits of fluff, would make sure they were all in classes together (to force the professor to slow down, since they far outnumbered the real students, and there was some taboo about failing 90% of a classroom). Even in their final years, they'd routinely badger me with simple calculus or modular arithmetic questions.
As an aside, an analogy in the software world to the people on the "teaching track" are the people doing CS degrees just to get a get an accreditation to get a job. The sorts of people who manage an entire degree without knowing any theory -or- programming, and wind up being examples in the recent "how to filter out job applicants" thread.
Interestingly, I deliberately avoided CS department to avoid them, and ran into the same problem in mathematics. (But math still turned out to be a much better choice, because the student body was so small that they could run special topics courses on whatever I wanted.)
2. To become a "professional", you are required to go through an accreditation process, which assumes you are the sort of person from (1). (Not to mention the political problems with the process.) The process has nothing to do with teaching you to become a teacher and everything to do with saying the right things to the right people.
(3.) As Kit says, once you become a teacher, you are immune to market forces.
The fact of the matter is that smart (or even average-intelligence) people are heavily selected against, as are people who believe in personal responsibility or independent thought. If you somehow get through, you are not allowed to use any of these traits. You won't be allowed to do anything creative or useful. (Almost) nobody around you will be capable of doing anything creative or useful.
That's why I don't believe I'm a special case in being more qualified than professional teachers -- pretty much anybody would be. And as you say, it takes an enormous amount of time and dedication to do, which gives a positive selection effect to home-schoolers.
(Also, I am in Canada, not the US, though from what I hear, the situations are similar. I say this because there are a lot of problems that seem to only occur in the States (for very States-specific reasons), and this is not one of them.)