I do not personally know the numbers on that, but going by what you state here, it sure doesn’t seem like “so many.” In fact, it seems like a pretty small minority.
It generally is. No one can stop a determined student from applying to med school. But it would be foolish for a weak student to put all their eggs in the med school basket when their chance at acceptance is incredibly low. An advisor strongly encouraging consideration of an alternative career path is the responsible thing to do. They would be quite remiss in their duty if they did not do this.