Why do some people not consider biracials black?

I think if people are 50% black they should be considered black. they dont look white or the other race they are 90% of the time so why go out the way to say they arent black when they are. Also people think people that are a light caramel color and have curly hair are mixed and not black, but my sister has light skin and curly hair and both are parents are black..Biracial people can come out with any hair type of skin color as blacks so why try to seperate them. i know biracials my skin color im a dark carmel..and some even darker. A black person with two parents can come out looking "mixed" so i mean wtf? people wanna say obama isnt black but i have cousins that resemble him with two black parents..so whats up with that? and here are some celebrities with two black parents that look "mixed" vannesa L. williams and the singer Pebbles.