Here's why:
* The Cornucopia is a dangerous place: The tributes fight fiercely to grab supplies at the cornucopia. Hiding there would make you an easy target.
* It's a starting point: The cornucopia is where the games begin. Tributes run away from it, not hide in it.
If you want to find out who might be hiding in the cornucopia, I suggest you refer back to the book or watch the movie again. You'll find the answer there!