They keep it hidden for several reasons, though in reality they boil down trying to pay you as little as possible.

-They hope to wow you with the position so you'll accept a pay lower than you wanted. This doesn't work when the salary range is listed up front and is so low you won't bother to respond.

-They're going to base their offer on how much you make now. This works when you're trying to move up in your career, they can offer you a little more then you make now but not as much as if you're already getting paid well. Again, they lose this position if they're up front with their range.