This is because of white supremacy spread by Europeans over the last 500 years. There was a time when "Arabs" used to write books on the Superiority of Black People over whites. I even made a thread about it:
https://www.thecoli.com/threads/did...the-superiority-of-blacks-over-whites.553137/
There was also a time when white skin was the sign of a slave in "Arab" society:
Mamluk - Wikipedia
The word Mamluk is Arabic used to describe Muslims of slave origin. And the mark of slave origin at that time was white skin. If you've seen any image of a Mamluk it's that of a white person. This is in direct contrast to the Arabs who at that time were defined by their black skin. And in most pictures were depicted as being black.
Here is a picture that shows what I'm saying:
The description below describes the two white individuals in this picture as Mamluks while it describes the black man as a "Bedouin". Bedouin was a term used to describe the pure Arab desert dweller.
The painter of the this picture is clearly telling us that pure Arabs (aka Bedouins) were of a different race than the people called Mamluks. The Mamluks were white while the pure Arab was black.
So you see, there was nothing about the original Arab culture that was anti-black. It was actually very pro-black. What happened was over time the white Muslims of slave origin began to outnumber the black Arabs and that coupled with the spread of white supremacy by Europeans is what has made the modern Arab culture anti-black.
Study history and you'll realize why the world is the way it is today rather than just believing this is how its always been.