If we are all equal, why do the words stupid and smart exist?

Clearly, plenty of people are pretty stupid.
And a few people are smart.
This is an accepted fact.
If it were not an accepted fact, then education, accomplishment, scholarship, the President etc. would not exist.
And yet in spite of the dichotomy of smart and stupid, there is a constant appeal to the notion that everybody is equal.
So which is it?
Are some people smart and others stupid?
Or are we all the same?