Let's talk evolution and go back a few million years – you'll understand better!
About 6.5 million years ago, the great apes and hominoids gave birth to the hominid lineage which includes modern gorillas, chimpanzees, bonobos and... humans! Hominids are classified as active carrying primates, which means babies cling to their mother who carries them. However, Homo sapiens sapiens (modern man) differs from its cousins in many ways, including bipedalism.
In the beginning, all hominids were quadrupeds until 4 million years ago, when Australopithecus Afarensis appeared – like Lucy, whose famous remains were discovered in Ethiopia in 1974. This species was the first hominid to walk upright.
The newborns of other hominids such as gorillas compare to a 3 month old human. Being further developed, we can suppose it makes it easier for the infant to cling to the adult, leaving the adult free use of its four members. Our ancestors, supposedly acting on instinct, naturally used their know-how to make baby carriers. They could carry their offspring securely while keeping their hands free to go on with their daily tasks.
History is a great teacher...