That's the thing though. It actually assumes ethnic groups of different peoples, are exactly the same and no different from each other. The individual is always a very tiny minority(less than 1%) within an ethnic group. The individual often does not share too many cultural, and behaviors similarities with the other members of their group but they still retain some behaviors, and culture from their group.

Let's say someone says that all white people are racist(a black person), but in this context what do they mean? Have they ever faced discrimination in their lives? Have they ever been rejected by a white person for being black?

Let's say someone says that all black people are stupid and lazy(a southern person), but in this content what do they mean? Have they ever witnessed a stupid and lazy black person in their lives? Have they ever had bad work experiences with black people?