Why do they tell kids "you can be anything you want"?

It obviously isn't true for all kids. Some people are just naturally more intelligent than others. Are they referring to the best and brightest when they say that? Not all kids have enough intelligence to become doctors. I've seen a lot of kids who aspire to greatness when they are little only to have their hopes dashed as they grow older and realize their limitations. Of course there are some who really are intelligent enough, but many are not. When teachers make the blanket-statement "you can do anything you want" it's just not true for everyone. Wouldn't it be better not to get their hopes up? They could say something like "some of you might be smart enough to become doctors someday" but why do we lie and make them think that they all are? It's only going to lead to disappointment for many of them.