Question

why cant a baby talk when he is born

Answer 1

First, the “mental developmental hypothesis” states that one-year-olds speak in babytalk because their immature brains can't handle adult speech. ... Likewise, they don't speak multi-word sentences or use word endings and function words (“Mommy opened the boxes”) before their brains are ready

Answer 2

When a baby born he don't know what to do, what to say as I don't know any alphabets....
He is new in the world they are people talk by the time we learn to talk......