Because if they didn't show black people in the movies in America, it would be considered racism, bigotry, and the worst violation of civil rights that one could ever perpetrate. Therefore studios find black people to fill such roles so that they don't have to deal with the backlash of not having black people in movies like this.
Even CDPR has been criticized to no end for not having more ethnic, or black people in the Witcher 3. Therefore, when Netflix made The Witcher series, they made sure to put a bunch of ethnic characters in the show, even if the show is about European history, and lore. But, they have to do it. I know, it's idiotic. Imagine if there was a game, or show that focused on pre-colonial sub-sahara African history and lore. And, in this show there were a bunch of white and Asian actors representing characters in black history. Yup. That would make a lot sense... In fact, there would be hoards of blacks and Africans that would cry fowl as to how inaccurate that historical representation would be. That would be valid. But when the white man wants to make a show about his/her own cultural heritage/history, that doesn't have black people in it, that's considered to be racist.
I love every races. And I never understand those people hating races and prefer their own races. IDIOT!
Anyway.. Because if they didn't show black people much then it is obviously racism and civil rights. The black people should be seen often in the movies. Hate exists and this is the idiotic world we live in today. They have no brains! Love doesn't exists to mostly people. That's why.
To be honest, I have seen many black people in the movies like Boyz the Hood and etc.. Hate those movies.. Heartless and directors were and still perverts today because of this! I called them racism because they labels black people as a bad people because of gangs in the movie. Those black actors willing to do that in the movies are greedy for money and forgot about their skins are the jerks on Earth! They should refused to be part of it due that it will be bad influence in real world. And it does! Sad!