There's a lot more to language learning than being a "sponge". Virtually all the grammar we learn is productive/ creative--that is, we apply it to new words, and say things we never heard anyone say before. And the grammar is implicit in what we hear, so children need to extract it in a form that can be generalized to new thoughts and words.

>Virtually all the grammar we learn is productive/ creative--that is, we apply it to new words, and say things we never heard anyone say before.

That's downstream of the sponge phase. So much so, that initially we only absorb and don't talk yet.

This is why learning Latin the way I did (very methodically and technically, with no real speaking/responding) makes you good at parsing it, but not at speaking it. There are schools today where it's taught as if it were a spoken language.