I took three years of Spanish and got an A every semester. Even when it was still fresh in my mind, I was nowhere near able to hold even a very simple conversation. And now just a few years later it's all totally gone from my brain.
My mother's native language is Spanish and she never taught me, which I resent her for. But I still find it incredible how shitty my public school education in Spanish was. We really should be teaching kids a second language from kindergarten up.
It's the same as every other high school subject. It exists to create a bare minimum level of skill widespread across society. Just like Math or Chemistry, you can follow up on it afterward and become proficient at it. You can even make a career out of it. But most will forget it (because they chose something else instead).