Why Are there So Many Languages?

An acquaintance of mine speaks Xhosa, one of the click languages of South Africa. It is strange-sounding indeed. There are so many languages and they all seem so arbitrary. Imagine if people differed from each other in configuration of eyes and limbs, to the extent that Xhosa differs from English. It doesn’t seem reasonable that there should be so many mutually unintelligible languages when human beings are so much the same in so many other ways. Why can't we all understand each other?