Because it results in the addition of token characters just to satisfy some diversity requirement. So what if there are few playable black people?
Again with this idea that white people somehow naturally appear in video games as if they grow from video game white people trees, while putting minority characters in is some big extra effort to "satisfy a diversity requirement".
All video game characters are made from scratch. You don't get white people for free, and then have to make a decision to put minorities in. You make decisions, every time you create a character, about whether you're going to make them male or female, young or old, white or black, etc.
If there are white people in a game, it's because at some point a creator decided, "this character should be white." If there are black people in a game, it's because at some point a creator decided, "this character should be black." This idea that white characters just show up on "naturally" and minority characters are some token things you have to go out of your way to include is
stupid.
It's idiotic, and there's no way around that.
When people ask for diversity in games, they aren't saying "take this thing you never have to think about and make decisions about it." They're saying, "you make this decision every time you create a character. Maybe don't make the decision to make a white character every single time."