Why? They're (a majority of them) are in exactly that age when the great minds can influence them most strongly. Later on in their career they'd have a bunch of sticky prejudices, their own solidified research interests, and so on. Meeting a great mind won't sway them as much as it can do to a young and unspoiled.
You know, it's funny; I was an undergrad when, broken-hearted over a girl and wondering why it didn't work out, I came across a partial explanation (http://paulgraham.com/nerds.html), thought, "This guy is smart and writes well," read some more (http://paulgraham.com/avg.html), and...well, six years of lisp later...here I am.
