As a spouse of a teacher, (isn't THAT a credential!) let me weigh in on this.
Yes, there are some kids who are "so dumb" that they can't learn what most of us do. These kids are usually handicapped in some way. These kids get special classes to make sure they learn enough, in a way that's meaningful to them, so they can function their best in society. They learn how to go to the grocery store, count money, read a bus schedule, etc.
These classes are called "Life Education" and specialized teachers usually need to teach them.
You also have other learning disabled kids, like autistic kids (my wife's specialty), reading disabilities, math disabilities, etc.
Teachers are professionals, much like architects, computer programmers, construction contractors, and the like. They are required to stay abreast of all sorts of studies, techniques, and take workshops to learn the best way to teach your children.
You know what happens when you have a crappy architect design something? You get a tacoma-narrows bridge.
You know what happens when you have a crappy computer programmer? You get Windows. (har!)
Would you want your house worked on by just anyone? Or would you hire a professional?
Wouldn't you want your child taught by a professional? Don't you care enough about your child to give him or her the very best?
I know that if/when I have a child, I want that child educated by a professional, not someone who'se faking it (like I would be.)
If for whatever reason I did not trust the public education system, I would find some way to afford to pay to send them to a private school.
Anything less is criminal for my child.
Now having said all that, I agree with the fact that requiring a teaching cert for homeshcooling parents is overstepping the bounds.
I'd think that any good parent would DEMAND that anyone teaching their children be professional teachers, even if it was themselves.