Because at this point, a native English-speaker learning another European language makes about as much sense as a native Mandarin-speaker learning another Chinese language. Not quite, but almost! English is the language of the West just as Mandarin is the language of China and Arabic is the language of the Muslim world.
Why learn German when it's only useful in a few countries, and only if you speak it better than the Germans, Austrians, etc. speak English (which you never will)?
Why learn French when they will mock you and despise you for not speaking PERFECT unaccented Parisian French (which you never will)?
Also, you can't really blame Americans - you have to blame the American elite, who design our educational system so that language classes are only offered starting in high school (occasionally middle school) - far too late to begin learning a new language if you're ever going to speak it like a native, and when they continue to use ineffective rote learning methods? The American system is actually designed to discourage Americans from learning a foreign language!