The simplest answer is because our modern racial constructs are, well, constructs -- they are socially constructed. "Black" is a completely modern concept, which is rooted in the experience of the Atlantic slave trade but formalized in the 19th century. It would make no sense in the context of ancient Egypt to call people by a modern racial category.
For more on this, see this section of our FAQ, in particular these threads:
Monday Methods: "No but what race were the ancient Egyptians really?": Race as a concept in history by /u/commiespaceinvader
How would an Egyptian in Alexandria at the turn of the Millenium BCE have perceived race and skin tone? by /u/cleopatra_philopater
