True enough, we teach in every interaction, but I think you can see the marked difference between the experience of learning from observing a personal interaction and learning in a college course. What you get from a college course is quite a bit bigger and broader in scope than what we glean from a random encounter (save for the occasional epiphany). Moreover, putting oneself forward as a teacher in such a manner encourages the student to view them as an authority on the subject - perhaps not perfect, but still very accurate.
When this is not the case though, when a person is not very accurate and they have elected to actively teach others, those students then suffer from a great deal of misinformation, will make bad decisions based upon it, and may very well spread this misinformation to others.
In short, there's a very good reason why we don't let just any person teach in a college.