Because you're assuming the grammar is tied to the language, rather than the structure of the human brain. There may be some grammatical primitives that the bra

Because you're assuming the grammar is tied to the language, rather than the structure of the human brain. There may be some grammatical primitives that the brain structure supports which enables the grammar of language to function on top of them. This is one of the common linguistic explanations for why you can teach a human to understand grammar, but not a dog, for instance. Dogs can learn. They just can't learn grammar. Why? Because the human brain is primed with grammatical primitives that a dog brain hasn't got. Personally, I think that's a bunk explanation, but this conclusion is not obvious either way.