In that very same sense, "white" is not a race. There are many MORE whites deriving from many white countries. But they are still unified under the fact that even many white people at the bottom of the economic and social scale still see themselves as "better than blacks."
In terms of race among Latino/as...Most Mexicans (that is, people born and raised in the country called Mexico) are a mix of white and indigenous peoples from varying cultures and regions. There is a great deal of racism among white Mexicans toward the "brown" Mexicans.
White Americans (and, unfortunately, some black Americans) identify anyone who speaks Spanish and has brown skin as "Mexican." It's like Africa--a continent of many countries, but white people see it as a unified black (of varying shades) country and identity: African.
Of course racial identity is slippery precisely because most of it derives from color regardless of actual national origin.
Most Americans have very little acquaintance with Spanish Americans (i.e., people from Spain), who generally have white tones to their skin. So that can really confuse white Americans when a white group speaks Spanish exclusively. These "Mexicans" are not even from one of the two Latin American continents.
And then, the Portuguese...both in Portugal and in Brazil. Another mix that confuses white people. Tho Portuguese sounds very much like Spanish, so hey, they must be "Mexican."
And here I am using the word "American" when in fact everyone from Canada to the very tip of Brazil is "American."
Racial and ethnic identities are of course fluid and thus difficult to pin down with a specific, signular name.
That's just the nature of things.
It is NOT the nature of things for people in the United States to be so provincial in their understanding of race, racism, and the rest of the world.
In large part this is due to two unfortunate drawbacks of American education and media:
American K-12 education gives zero to a small portion of time to World history, geography, languages, and cultures. Even many if not most colleges don't include study of world histories and cultures in their required foundational courses.
Study of people outside of the USA is about as limited as the study of indigenous peoples and cultures IN the country. (Same for Canada, but not quite as bad.)
Even in the 1960s when core requirements were far more rigorous both in HS and college, there was very insufficient coverage of the rest of the world in schools. I was fortunate that I went to an "outlier" public school (Claremont, CA) that had special courses like Russian history as an elective.
AND at the same time, our MEDIA (including the "mainstream media") give very little attention to what is going on in the rest of the world unless it involves some conflict -- either direct or via one of opponents like Russia or China.
Bottom line, all of this comes down to a plethora of ignorance. AND to a desire to keep identities simple and easy so white people know who they are NOT and so they know whom to regard as enemy or potential enemy...even among one's own citizens.