Why dont black people have straight hair like white people?

What decides how wavy, fine or coarse a hair becomes is decided by the follicle, the hair sac, the place where the hair grows. Some of them produce curly, coarse hair, and some make fine, straight hair. The kind of follicle you have decides the kind of hair you get.

And for some reason, those who get dark skin usually also get the kind of follicles that produce coarse and curly hair.