They don't have to defend themselves against it. This makes them completely open to language input. Language must get in before it can come out. Language must be correctly heard before it can be correctly produced. That requires total openness.
Adults tend to have their defenses up.
They tend to protect themselves. As a result, some language input gets blocked or filtered out and never gets in. Children let life happen. Life is extremely complex, but not for kids. That also carries through when they go abroad.