because it rhymes,
why do people say "see you later, alligator" when they say goodbye? and then the person they're leaving says "in a while, crocodile?" i don't get it, why do we even need to mention alligators and crocodiles like that? what's wrong with just saying "see you later" or "bye"? let's stop with all this crazy animal talk when we're departing from each other, and just keep it simple.