You asked for an explanation as to why White people get defensive when Black people raise the issue of racism. I gave one. An explanation, not an excuse, be it noted.
It is, simply, a demonstration of how White, middle-class people are trained to see the world. Egocentrism, individualism, lack of empathy and a denial of any responsibility for anything except ones' own wellbeing. The culture informs the language, which in turn informs thought, which then reinforces the understanding of language within that culture.
Thus if you make a statement which is intended to be critical of some aspect of society, it cannot be heard in that way. The culture rejects the concept of society in any but an economic sense. Therefore, any criticism not levelled at economic processes and structures can only be understood by such a person as a criticism of the individual, themselves.
It is of a piece with this that such people will proclaim, with varying degrees of complacency or stridency: "I am not racist.", "I don't judge on colour.", "I have Black friends.", and consider that sufficient. They are unable to grasp that they are in fact part of a larger social entity which is structurally and systemically racist. The notion that they have a responsibility to accept this and change it, collectively, is something their re-coded language does not permit their circumscribed thought to even formulate, much less accept. Hence the false definition of collective action as 'rioting', no matter how peaceful.