"Common sense is not so common."
We have to acquire common sense with uncommon efforts!
At the age of 30, if you have acquired and retained some "common sense" in most of the subjects you study in primary and secondary school, you are a person not so common!
We learn and we forget, that is the reason for us to keep learning and retaining what we had already acquired. If we can build something on top of what we retain, we can be name "expert" in that particular field.
It is that simple.
Yet, simple things are not easy to acquired!