Teacher's core competency is teaching. They study it for years, practice it for years, in a system tuned over time. You have never done it before. You really think you can do better?
You're overstating most teachers' competency, not to mention, most of grade school is pretty damn basic... Most commentators on this site are more educated than the typical teacher.
Not to mention, the difference between teaching a class of 30 versus only 1-3 children.
I do question that having a single teacher for 20+ students teaching all subjects for the entire school year is obviously going to work better than motivated homeschooling parent(s). While teachers do have formal education on the matter, it also includes how to manage many children in a classroom setting and how to interface with schools and their processes. The part of the education pertaining to the subjects teachers teach is actually pretty incomplete. To teach elementary students, you have a major, but still teach them subjects that you might only have an average understanding of (like a history major teaching a music class).
Given all that, it doesn't seem hard to believe that parents could do better for a single child than pre-school and elementary school teachers.
As a child gets older their needs evolve such that no one person has everything required to best support that child. That is an advantage of schools (usually high school), but could also be solved with tutoring and other programs available to homeschoolers.
Teachers are babysitters first and foremost.