How old were you when you first "learned" French? I've observed that kids/teenagers who aren't actually interested in learning the language retain just enough to pass their classes and then it all just drifts away. I was the same "learning" Spanish in high school in Texas.

Until I moved to a new country and _wanted_ to learn the language, I could barely remember how to ask where the toilet was. Now that I'm invested and interested, things are much more sticky.

I believe this is why much of the world has latched on to learning English. There is so much content available that people _want_ to consume, that it becomes a hobby they are actually interested in, rather than a chore. As more and more people learn English, it becomes a positive feedback loop of content creation that nearly the entire planet can participate in.