If everyone did speak the same language, it wouild over a few centuries re-fragment into multiple languages again. 300 years ago, all English speakers spoke pretty much the same language, and all could be clearly understood by each other. But now when you dial an 800-number, your get a person who speaks perfect English but you can't understand him.

In my lifetime, a whole new language has split off from English and begun its own separate evolution, called Ebonics. A new language was similarly created a few generations earlier on Caribbean islands.

A thousand years ago, the people of Germany and the Netherlands spoke the same language, but now they are no longer mutually comprehensible. Those same Dutch went to South Africa, and now the Dutch spoken there is not always intelligible those back in Amsterdam.

In many cases, new languages separated and evolved along their own trajectories because subgroups of people wanted to separate themselves culturally from those speaking the mother tongue. The more you try to impose your ideals on everyone else, the greater the desire of people to want to distinguish themselves from you, and they do so by creating their own separate language.