It is not the concept knowledge that is lacking but the pedagogical knowledge. Knowing how to teach something is different from just knowing the thing. Especially when you consider that your child will have different learning needs than you did. Teaching professionals are experts at what they do.
A teacher has an entire classroom of children who rotate periodically, all of whom have different skills and learning styles.
A parent teaching their child for months and years gets a pretty good idea of how their child learns. They don't need to know how to accommodate every learning style, they just need to figure out one.
I don’t mean learning styles particular to students. I mean something specific to what is being taught: the dozens of different ways of teaching each skill.
This is quite an important point, though I would hope that someone who decides to dedicate 10 years or so of their life to the task would read up on the topic. (I mean that non-ironically, I suspect it is often lacking and doesn't happen.)
On the flipside, I have had several real teachers about whom I frequently wondered why they had chosen teaching as a profession. It was like they lacked both the aptitude for it, and had an intense dislike of students. To be fair, most were not like this, and there were a few truly excellent ones. But to make the argument that they are all highly trained, excellent professions is a bit of a stretch.
It's part of why teachers now generally stay in particular grade levels, rather than following the students or teaching multiple levels as was once standard. They become experts on the pedagogical content knowledge of their particular level, so they can apply years of experience to solving the unique challenges each kid has.
