After you finish an undergraduate degree, nobody wants to hear about how you were an exceptional high school student.
And honestly, if you won a medal in the IMO and don't have anything to show for your undergraduate years aside from a high GPA, there's something wrong with you.
Though it works that way for the rest of your life too - if you sucked at your last job, nobody wants to hear about how you were exceptional in college. And conversely, if you last worked on an exceptional project, nobody cares if you dropped out of high school.
And honestly, if you won a medal in the IMO and don't have anything to show for your undergraduate years aside from a high GPA, there's something wrong with you.