Joke answer: "Because like an adult asking a child, 'What do you want to be when they grow up?', they're actually looking for ideas because they don't know what the hell they're doing":)
Should my boss answer like that, I would be very offended, since I'm a fairly senior developer who doesn't want to manage people and thinks he knows where he's going. If he said that referred to the HR intern, well, he could be right (still offensive, but sometimes right) :)