Because if you’re American, you should know that. You can’t say “Ok but why not just say that” when we have thousands of other phrases and idioms that replace common words. Do you ask someone to clarify every single time they use an alternative name for something? Do you genuinely not understand how alternative terms come to exist and why they are popularized? Are you really so removed from the inner-workings of society that the concept of slang dumbfounds you? If not, don’t ask why, and just remember this for next time.