Because our society is a culturally white dominant society. It's considered good to be white, whiteness is associated with success, and non-white people are seen as "the good ones" if they appear to behave more white. People with strong racial bias tend to assume that a non-white person who appears "less white" (has an accent, dresses a certain way, etc) is less educated, less culturally attuned to American norms, and needs things to be explained to them regardless of whether or not they actually do.

I agree with you that racism is not limited to white against all others, nor are all white people racist, or all racists are white. However, I would argue that it is not an invention, but an actual thing that people experience, and enough people experience it that there is ongoing discussion about it online and in-person where people immediately recognize the meaning of the term.

The use of white in whitesplaining is imperfect, yes. It is absolutely true that a racist, say, Asian person would talk down to a black person. The issue of racial hierarchy in the US is complicated by a long history of colonialism and paternalism that taught non-white people that being non-white is bad, and being white or as near to white as you can get is good. By being part of predominantly white American society, non-white people have adopted this pro-white bias that looks down on dark skinned, accented, "ethnically" dressed people within their own ethnic groups. The "good ones" partially identify with the group of highest regard, white people, despite white people probably regarding them as broadly non-white.

Can I also add that a discussion and/or acknowledgement of whitesplaining is not an excuse to hate white people. When we (both non-white and white) people discuss issues of race, racism, and racial inequality it is to call attention to our identities and experiences, not to incite hate. Pointing out differences is not hateful, nor is exorcising past hurt. Some people are angry about how they and their ancestors were treated. I think it's fair for them to feel that way. Would I prefer to keep all discussion civil? Yes. Some people feel differently, but we cannot paint this discussion with a broad brush and dismiss it just because it's uncomfortable. Calling out the problems of the past, of examining the imperfections of history and current practice, is not hateful. The only way we make progress is to identify problems so that we can figure out how to solve them.

When a person of any background exhibits racial bias and talks down to someone for being lower down the "racial hierarchy," it is a result of a lifetime of cultural norms, personal experiences or lack thereof, and education or lack thereof. Everybody, and I include myself in this, is guilty of racism some times because no one can be aware and sensitive to all racial issues and other groups' histories. It is unfortunate that the word racist has become such an extreme word that seems equal to evil. No one is perfect. Everyone makes mistakes. Not every racist act is intentional, but it is still harmful. Hopefully instead of outright rejecting uncomfortable discussions of race, of saying there is no problem when there clearly is one in this country because it's the easier thing to do, we can examine these issues with open minds so we weaken our biases and diminish the damaging effects of racism from our past.