Um... studied eight years for a four year degree? That's for high school chemistry teacher. Eight years is doctorate level.
In teaching below college level, there's maybe more flexibility than some realize. Depending on the state, a teacher must be certified, and states have different requirements. But I've seen family members who majored in one course suddenly find themselves assigned to teach another. Again, depending on the state, they have to get a certificate in that course, but it's still not their major. Maybe that's how coaches end up assigned to the most surprising classes.
From what I've observed from the outside looking in, homeschool curriculum can be very solid, with the teacher's material designed to get the teacher up to speed. What I saw advanced with the course. Granted it's a mixed bag, but, unfortunately, so are public schools. I had a low opinion of a history book ours had back in the day due to watered down content and errors, and did not think much of the "estimate" craze. The "estimate" craze had the idea of teaching children to estimate, which is actually good, but before they knew how to calculate the same thing. In other words, a guess without any basis of whether it was even in the ballpark. When I asked one's teacher about that, her replay was "It's in the standards."
That's why I mentioned AP courses in particular...
Many (if not most) school districts have higher requirements for those, that don't just end with the conferring of the degree. Virtually nobody's first teaching job is a 12th grade AP Chemistry or Bio course.
Teachers cannot obtain an AP certification online or in-person. Instead, they can complete the AP Course Audit to have their courses authorized and gain access to AP classroom resources, such as practice exams for their students.
work.chron.com
Homeschool curriculums (in terms of reading materials) can be equivalent, but the environment and hands-on stuff is not...neither are the labs for science courses.
Just one example, in my high school bio class, we did a dissection of a pig and a cat, followed by delving into (by getting a first hand look) at the differences in the cardiovascular and digestive systems.
Is that happening in homeschooling environments?
The odds of a child having 2 parents, that between them, can cover the territory of a history major, English Lit major, math major, science major, PE/Health teacher, shop teacher, counselor, foreign language teacher, and computer science major, that know enough to teach it at a high school level (and have the related equipment - a gym, a wood/metal tech shop/etc..), are quite slim.
There's a reason why as students progress from elementary to high school, the teaching staffing transitions from "shallow breadth" to "narrow depth".
There's plenty of adults that can handle teaching everything at a 2nd grade level, teaching it all at a 12th grade level is a different beast.