Simply knowing lots about one's subject doesn't make one better able to teach it. I've had many teachers and professors over the years who are brilliant, but have no idea how to pass their knowledge on to others.
There are amazing teachers who teach because they are passionate about helping kids learn how to learn, regardless of the subject matter. They are always learning and expanding their knowledge base.
There are also terrible teachers who are lazy, don't know what they are teaching and don't belong in a classroom.
Assuming all teachers do so because they "can't do" or aren't as qualified because they aren't experts in the field they are teaching is ludicrous. An elementary school teacher isn't going to be better at teaching kids to learn how to read and write if they have a masters in English.