learning a language is NOT about vocabulary or grammar rules
I think we've got it all wrong. Learning a language isn't about memorizing vocabulary or grammar rules.
I was lucky enough to have learned a bit of Spanish while traveling in Latin America. When I was talking with the people there, I realized that they know a lot more words in English than I know in Spanish; for instance, they knew the English word for "blueberry" but I had no clue what it was in Spanish. But they also couldn't put together a complete sentence in English the same way that I can sometimes convey ideas in Spanish when I need to.
Then it dawned on me. I've been learning naturally, in ideas (and unconsciously so). Instead of learning a word and its meaning and its conjugations, I learned a phrase. For example, I learned "tengo hambre" for "I'm hungry", instead of learning "tener" for "to have", "tengo" for the conjugation "I have", and "hambre" for "hunger". This appears to be a slower way of learning: in theory, once I've learned "tengo" means "I have", I would be able to use it in all other possible phrases by putting it together with all the other words that I know. However, in reality, I found that I never use the words that I learned alone, and that I was never able to apply all the grammar rules that I memorized.
And it makes sense - think about how native speakers learned the language. They weren't born with perfect mastery of the language. They learned through ideas as well - they had to express "I'm hungry", but didn't know how, so they learned the phrase; or they heard the phrase and internalized the meaning. Most native speakers don't know all the grammar rules that language learners know - they know that it sounds/reads natural to them.
But this is not the case in our educational system. We're forced to memorize vocabulary and grammar rules, and exams focus on this as well (perhaps because it is the easiest way to test how well you know a language). But if our final goal is to learn the language, there is no shortcut. We need to do it through repetition, through listening, speaking, and internalizing ideas rather than words or rules.