I think the phenomenon of few multilingual people in the Anglosphere is the result of little actual social necessity or benefit from actually conversing in another language in everyday life, making it both more useless and more difficult to acquire. And they have good company as education subjects that the market doesn't value much that then acquire a cultural premium: with fine art and philosophy.

I think this becomes more acute when one's profession is specialized and the professional circle doesn't value multilingualism, especially because of how much harder it is to become exposed to the genre/technical writing and jargon of your professional language in another language, as opposed to mundane professions.