Why don’t most Americans learn a second language? Plenty of well-informed answers there.
Can’t? Last I checked, and contrary to international popular opinion, Americans are still human, and capable of learning other languages. Not as driven to learn a second language as a Serb or a Greek? Well, obviously. That’s one of the benefits of being a hegemon. A benefit and a curse.
Lucian of Samosata was the premier author in Greek of the 2nd century AD. And he was proud to be Syrian. How many Greeks were learning Syriac in his day?