I don’t know that it’s necessarily that it’s “inefficient”. Moreso that it’s difficult for a language to actually stick and be useful if you’re not immersing yourself in that language. You can go to class all you want, but if you’re not trying to actively immerse yourself in it beyond class, you’re not going to learn the language no matter how good the teacher is.
It’s relatively “easy” to immerse yourself in English language content because English has sort of become the “lingua Franca” of the modern world. Something like Polish, for example, isn’t.