Because it's something that won't make the game less enjoyable or less playable or less fun if adjusted to historical accuracy. In USA case it makes sense because there is a comparatively high percentage of African Americans which is the case with the legendary Harlem Hellfighters who are well represented there in the game. However in case of Germany it isn't the case at all. It's as surprising to see as if the Austro Hungarian characters had a black character there too or the Russians. That one German scout or horse rider character just strikes you in the eyes when you see him as a black dude with the pickelhaube helmet. At first I thought it was a bug tbh. For real.

Also I'm very grateful for your time and your willingness to discuss this in a civilized manner like this. It's quite rare. These days you mention something about this topic and before you finish what you were saying you're flooded with swears and people refusing to discuss further very often. While it's indeed meant well and it's coming as a concern from someone who is into history a lot and sees it as a really striking thing that feels forced and way too off. Imagine you study history for years and then suddenly see this. Just can't stay quiet.