Americans are not stupid. People, in general, are not stupid.
Think of all the great intellectuals, inventors and artists that America has contributed to the world.
Thomas Jefferson, James Madison, Abraham Lincoln, Edgar Allen Poe, Emerson, Thoreau, Thomas Edison, the Wright Brothers, JFK.
And the ones who came to America, attracted by its egalitarian and humanitarian spirit.
From Einstein to Pinker.
Many of whom were multilingual, not to mention geniuses.
Calling people "stupid" is what causes cultural misunderstanding, and by logical extension, world wars.
With all your linguistic getting, get some understanding. Otherwise, you will spend your entire life imagining that you are a superior sheet, having never grasped the essence of an ongoing problem that has cost billions of lives and frustrated the advance of humankind since Time Immemorial.
It is true, America is largely a monolingual nation. There are many possible explanations, but the principal one is probably cultural isolation. Folks generally develop polyglottal language skills as a result of cultural impetus, and necessity. Europe, for example, is a veritable stew of languages, so folks gifted with bilingual and trilingual skills are not unusual there. (Of course, European languages are relatively close in etymology, so once you know French, for example, Spanish and Italian are relatively easy to acquire.) Since Time Immemorial, folks have understood that a Lingua Franca (at one time French), was necessary to facilitate cross-cultural communication and mercantile activities. Americans are blessed, because the Lingua Franca of the world is increasingly English - largely, I suppose, because it is, although complicated, relatively simple, as world languages go. There is a running joke in the linguistic community. "Americans are very smart - they can speak three languages - English, English and English." It is a tired old meme, but there is great truth underlying the humor. "C'est la vie." :)