And exactly why does America speak English, when a few centuries ago it was filled with native speakers of many other languages that are now very rare today?

Because the British Empire (and the Spanish and French empires) killed a great many of them, and forcibly imported their own cultures, people, and languages.

Which is unquestionably imperialism, and undoubtedly cultural. Hence: cultural imperialism.

If it weren't for cultural imperalism, the only native English speakers would be the population of the UK, which is about on par with Germany.

;-)