"Not ALL white people" is a direct product of the "individuality" enjoyed by ALL white people.
This means you have the luxury of being "one person." I will explain for a bit of historical context: Before the 60s, white people actually identified as a "collective" of "whiteness," and of whiteness of "Man" in particular. After the Civil Rights era of the 60s, the collective view was replaced by a cover of "blindness" and terms like "normal," "neutral," and "natural," and "nude" were used instead to subtly identify white. Think bandages, hosiery, behavior, language, etc. Simultaneously, the "Me" movement of the 70s created the notion of "individual," as in responsibility, accountability, and identity. White Racist America had now effectively hidden collective responsibility and accountability for its racist past, i.e., it had become "Post Racial," and white individual identity became "normal," "neutral," and "natural."
Institutional racism was effectively hidden and interpersonal, individualized behavior was now defined as "racist" or not "racist."
Black people, on the other hand, never had this privilege of individuality, even as we try to attain it and even as we generalize ourselves using the guise of unity (but that is another essay).
I will explain the reason in 4 words:
Unconscious Bias against Black.
It is so deeply ingrained in the mind that it extends on a spectrum, if you can imagine, from fear of "dark" spaces and places, "darkness" as in behavior, all the way to the dark of human skin.
Racial antipathy lives within in each and every white person whether ALL white people want to believe it or not. You don't realize it exists because you have been conditioned to accept "individual" expressions of racism as "overtly abhorrent behavior" of which you may not abide.
But, you *group* us together, you generalize us, almost instinctively. Think "minorities," "people of color," and all the other "socially acceptable" terms used to encapsulate us.
Where are the "socially acceptable" words used to encapsulate you... other than white? Oh, yes. There's "normal, neutral and natural." Then there's "regular," and "classical," among others.
How quickly do you write "not all Black people" to provide a modicum of individual cover?
In essence, this generalization means, "What one Black person does, all Black people do, or are capable of doing."
There is a caveat, however, since in order for the ideology of white superiority to be sustained in the unconscious, the Black person must be inherently bad and inferior. This, too, is deeply ingrained.
Black people live in a perpetual ALL status from which we struggle daily to differentiate ourselves. It is why we vehemently try to distance ourselves as individuals from the Omarosas, Cosbys, R. Kellys and Jussie Smollett’s -- not even the perception of wrongdoing allows for escape from being ALL the same.
But white people can be the "lone wolf" EVERY TIME. You are never considered collectively responsible.
Therefore, when white people say "not *all* white people," you are exercising your *privilege* as that lone wolf, not accountable, not responsible.
But you should be.
Understanding that you are all accountable is key to your understanding the pervasiveness of racism.