I don’t know exactly why, but I can give you a little more information about the phenomenon. The tone of voice adults use when talking to babies is formally known as IDT, for Infant-Directed Talk. Informally, it’s known as Motherese or Parentese. It occurs in almost all cultures and languages, and therefore seems to be “innate”, whatever that means. Studies have shown that babies listen more carefully to adults who are speaking in this manner, and some people think that this aids cognitive development of infants. More here