I guess it's because there is much more to learning languages than having lessons at school. I've had both English and German lessons at school(I'm Polish) since I was 7 all the way to the end of school at the age of 19. So I had 12 years of mandatory education in both languages. Yet my English is completely fluent, while I can't say anything in German. All media I've consumed was always in English, games, films, books, comics - while I never had any reason to use German.

That's why I think that just having language lessons is in no way enough, you also need to have a reason to use a language to actually remember anything.