Because of the first amendment, and how the courts have very consistently interpreted to allow hate speech.
>and how has the US managed to do so well for centuries with it being perfectly legal?
If by "do so well for centuries" you mean the US's economic output and world status over the centuries, I would argue that profiting off of Europe rebuilding itself after two world wars probably outweighed the detrimental effects of hate speech (among probably several dozen other reasons).
If you mean "how has the US done so well handling the negative effects of hate speech for centuries without making it illegal", I would argue that hate speech has contributed to some of the most shameful and barbaric social dynamics over the centuries, and the US is historically well behind other modern countries on this front.
Maliciously categorizing someone based on skin color, race, income, social status and religion in an attempt to denounce hate speech.
Poetic
Because we've generally dealt with that by deplatforming that sort of speech socially and/or in the private sector.
The history of the civil rights movement is filled with boycotts and other sorts of social pressure campaigns.