That is an insane thing to say

I've lived in some white and brown parts of Canada, and currently white and latino part of US, and I've seen tons of black people, as well as asians, on top of the two biggest groups of each area.
It may be 13% but that's still like 40+ million people, they ain't just all huddling up in Africa or ghettos dude
Now to answer the Machete's question, dev will just put whatever kinda character they want in the story, there's no diversity check or quota they need to fill. If they wanted to put more they would've, doesn't matter if for plot purposes or just cuz they wanted another hot black chick.
The question is like asking "why does so and so game have no black girls," or "why does so and so game have x amount of black girls"
Cuz the dev felt like it, tis all it is.