While its true that slavery preceded the births of these men and they were born into the culture, their slaves were still men and women, and anybody who looked could see that. Sorry, but it wasn't obvious. It was so Not Obvious, that Abraham Lincoln disagreed with you about the social and intellectual equality of blacks. There was no anthropological knowledge base to sustain such an opinion that you think must have been obvious in its truth.
So you don't believe in God-given inalienable rights? Besides there were many slaves who could read and write. Just 'cause it was Arabic doesn't mean it didn't count.