Language lessons are good at giving you the grammatical foundation you need to become literate. I agree that they're pretty bad for fluency in spoken language. To be fair, though, language classes are usually 3-5 hours per week, and you won't become fluent if you move to France or somewhere and only spend 3-5 hours per week trying to speak the language.

Motivations matter too.

I took three years of German in high school and a semester in college. I could never get the der/die/das stuff to stick. Later on I started reading German fiction for fun and then I got frustrated because I know the verb but not who did what to whom and then I got a lot better in the grammatical agreement area quickly.

I have gone through many years of on and off anime fandom without formal schooling. When I do pick up a book about the Japanese language, I often think "that is obvious" or alternately realized I'd heard the phrase many times without understanding it.