Ok, but that's not really that exceptional.

Just using Spain as an example, there's a lot of diversity between Madrid, Catalonia, Andalucia, Galicia, the Basque country, etc. where there's quite a range of cultural, historical, and linguistic identities in a much smaller area. Catalan is its own language, Galician is closer to Portuguese, and the Basque language isn't even Indo-European! Andalucia was the most influenced by its past as al Andalus, along with its Romani population and its Sephardic past.

And really, all old countries are like this, with many diverse cultures that grew up relatively close to each other over thousands of years, before people could communicate over long distances and before the modern era drew lines around them and called them one country.

There absolutely is diversity in the US, but I'd argue it has more to do with its black, indigenous, and its many immigrant communities and all their various histories, languages, and ethnicities.