Because no one is bright yellow irl unless they're a few days from dying of cirrhosis.
Yellow is obviously much closer to lighter skin color than darker skin colors.
I always thought it was because of The Simpsons, but going as far back as the Tracy Ullman show, so that could have been an influence as far back as 1987.
How is it empowering? How long do you think a white employee could post with black colored emojis before someone said something?