This question already has an answer here:
Some popular examples:
Don't you know any better?
Now use that with out the contractions:
Do not you know any better?
Want to bet that many English speakers would find this weird or wrong sounding? The contraction-form is generally accepted by many by how it "sounds", but the non-contraction-form is deemed to be strange use. Furthermore:
Won't you come to bed? Will not you come to bed? Wasn't he lying? Was not he lying? Can't it be funny? Can not it be funny? Doesn't that offend? Does not that offend? Couldn't pigs fly? Could not pigs fly?
People, I've heard at least, argue that it's wrong used this way (should be, "Could pigs not fly"), so why is it "right" in contraction-form and "wrong" in non-contraction-form?