Why do people LOOOVE to say "not all" and pretend all groups of people are exactly the same?

For example, men commit more crime. As do black people. Black men commit the most crime statistically by far. Why do you feel the need to point out Asian women also commit crime as if it's equally as frequent? Are you stupid?
Why do people LOOOVE to say "not all" and pretend all groups of people are exactly the same?
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