Question about slavery?

im sure if youve read some of my previous questions and answers you know my position on the us civil war, but im not here to debate that today, what im here to ask is why is it that some people act as if the united states is the only nation to have ever had slaves, now i know slavery was a terrible thing but im tired of people actin like the us is the only country that ever had it, and im tired of people ociating the confederate battle flag with slavery and racism, it only existed under the confderacy for barely even 4 yours, but it existed under the us flag for much longer, and under many others, so my question here is why do people always ociate slavery with the confederacy and act as if the us is the only nation to have ever had it.