In an ideal world, it would only return one thing, which tells you exactly what you want.
In Google's ideal world, it would also only return one thing, which tells you exactly what they want to tell you.
Because let's face it, one of the biggest challenges with that idea isn't that there are tons of different valid answers to a question (though that could be the case for many political queries or ones related to art/media quality), but that hundreds or thousands of pages and sites contain the requested info, and search engines have to rank them in some sort of order.