for the same reason that a cat reared by a dog does not start barking, or a monkey raised by humans doesn't start saying "hello" - the animals are built differently so make different sounds, its a matter of anatomy rather than learning
Birds don't learn to sing from their parents Fern, It's Instinct.
A wrens nest is always built the same way and of the same material, a domed shaped structure with the entrance hole in the side of the nest.
How does it know? No bird is shown how to build a nest by it's parents and yet all species always build their nests just like their ancestors have done for millennia. It's instinct.
But hand reared parrots (and other birds) talk because they initate us. And did anyone read the story about the baby elephant who copies the sound of trucks?
All very good answers. Particularly like Trillipse's. I have a good mind to ask how the female cuckoo can lay different coloured eggs identical to the ones in the 'host nest', but I won't. lol