This applies equally to paid teachers, along with numerous downsides that don't apply to parents (i.e. being able to tailor education to a single individual, developing a relationship that lasts close to two decades, ability to slow down and speed up course material where necessary, and more). Paid teachers, contrary to semi-popular mythology, are not special and don't do anything that an average person couldn't do (they are not extra-"competent"). In the natural course of being a parent you learn how to interact, guide, and teach your children.

This argument also fails in many concrete situations. For example, where I grew up there is a decent homeschooling community made up of people with average levels of education, low to average income, and yet the kids perform very well academically and are well socialized. Saying that these parents are not competent because didn't get a badge (education-related degree) is absurd considering they do as well as the people who did get that badge.