But babies are sponges! As adults we can’t learn the same way!

The research is very clear in this respect, and it repeatedly shows that input is necessary even for adult learners. Not only that, but any studies that track improvements in spontaneous language production over long periods of time show that traditional language learning strategies don’t result in long-term benefits. Our personal experience is that as adults we can indeed acquire a language to the point in which we intuitively know which words, expressions and grammar sound right and which sound wrong. We have learned multiple languages to that point using exclusively input. Intuitively it also makes sense, since it would be really strange if our brain somehow had two completely different systems for acquiring a language, but both allowed us to reach the same end goal. No other kind of learning is available only to children but requires a completely different approach for adults to tackle.