I've been thinking about this for the last few days, but I couldn't get to a conclusion to why (most) people from the United States are so racist. Racism seems to be embedded in US's culture. I say that as a Brazilian that has already lived in the US and experiences the country's culture almost daily.
Addressed by First Sentence and second paragraph two some degree.
Brazil and the United States have very similar historical backgrounds. Both were inhabited by natives before the Europeans arrived in the first half of the 16th century and have a history of slavery, where in both countries it was abolished in the second half of the 19th century. Today, however, while Brazil is known for its miscegenation of natives, africans, europeans and asians, the United States seems to have mostly maintained the division that existed before the Emancipation Proclamation.
Addressed by third paragraph. Examples of explicit institutionalized racism (US ONLY) following and undercurrent of general class-ism include: Jim Crow (Segregation), Internment Camps, Racial Profiling (Stop and Frisk), Tuskegee syphilis, Philando Castile (and related shootings). While determining if the latter is racial motivated can be primarily subjective, that distinction is meaningless when considered from the perspective of coming from the institution itself i.e training.
For example: In Brazil, there are no such things as "black neighborhoods". Yes, in some places some ethnicities are more prevalent (that has to do with the lack of support the government gave to the ex-slaves once slavery was abolished and the recency of that event), but no one thinks of these places as "where black people live". There are no words that can only be used by one ethnicity, such as the N word in English. Also, "white culture" and "black culture" aren't really a thing. While rap in the US is mostly viewed as black culture, in Brazil it's simply culture. Some of the most important samba and funk singers are black, but that doesn't mean it's black culture. We see it as Brazilian culture.
Also addressed by the third paragraph. Institutional effects cannot be reversed to a significant degree in a reasonable amount of time by simply removing the cause. It needs to be counteracted. For example, a zoning law that creates a minority ghetto being removed will not result in integration or diversification on its own. You might have a couple of people who pull themselves out of poverty and then observers will ignore the causes of the condition in the first place.
I don't know, Brazil just seems much more integrated and patriotic than the United States in that regard (yes, I just said the United States is not very patriotic in a way).
I'm not aware of Brazil's history or governmental structure, but it may be the case that since the US is a federation of states, that integration can be limited to each state's individual history, culture, and governance.
This also has nothing to do with Democrats and Republicans. It has nothing to do with left or right-wing. It affects both sides. You know, all of those BuzzFeed videos such as "White Parents Who Raise Black Children" or "Things Our Black Moms Would Say" only exist because they think of black and white people as being fundamentally and culturally different. In Brazil, a video titled "Things Our Black Moms Would Say" would make no sense, because being black has nothing to do with the way you act.
I'm not calling you racist for being American. You are not guilty for doing things the only way you know to do them. I'm just trying to show you a different vision of the world and hopefully make you reflect about it.
EDIT: In this text, I used the word "racism" meaning "the act of diving people into races". It was the best word I could find to describe it.
@DonFerrari If your suggesting that I did not read the OP because of this EDIT, do you not think it likely that the OP was edited after I posted.
Irregardless, "the act of dividing people into races" is simply categorization. It is recognizing the differences between people. Even race is just a social construct to describe a group of characteristics that people share. For instance, it is not racist to say people of Jewish Descent are more likely to get Cancer or even Black People tend to have lower than average IQs.
Intent is what makes something racist, so in the latter example it is why you said such a thing (In this case, nearly every intent is racist, I personally can't think of a way other than here in this very post that isn't racist. [Intent is difficult to prove and subjective itself, so it could in fact be interpreted as racist anyway lol])
As a result, that categorization "the act of dividing people into races" is universal its not any more inherent in one country or another. You wouldn't even notice it to be able to describe it as racist unless that intent was behind it.