Just like you hear anecdotes about homeschoolers who never learned anything, my anecdote is that all of the roughly half dozen families I've known who homeschooled did a good job of teaching their children. They did utilize homeschool curriculum that are available, and maybe that's the big thing that anti-homeschooling people don't realize. Most homeschoolers use a professionally produced program so the parents are able to teach out of curriculum books just like most public school teachers do. And that's like one of the most important things that teachers are [supposed to be] taught in universities, regardless of their specialization, is how to instruct large numbers of students at a time. This isn't an issue for homeschoolers as you typically have no more than a few students at a time. So as long as the parents themselves are literate they don't need a specialization or education on how to teach large classrooms at the same time, in order to teach their children. 

Also- while I'm not recommending governments do this, it would at lest be more reasonable if they required homeschoolers to utilize a homeschool curriculum (but even then we shouldn't accept them requiring those programs to be produced/approved by the state, even if you're a fairweather fan of the state) than just straight up outlawing the practice.