I think it's just because hours spent learning by children often don't look like work to us. Like, when children are watching children's television or looking through those baby books with shapes and colors, they are studying. To them, that is learning. And I guarantee that in 1 hour of studying, I can learn every single color in Spanish, whereas a baby might need months of daily reading to finally understand it. But because we don't register it as studying, we would still say the baby learned language "effortlessly", while adults need to "study".