People speak different languages because a long time ago more than one group of people figured out a way to communicate with one another, and they weren't living close enough to influence each other (thus creating a merged language). Also, because language evolves on a regional level. If we didn't have television and an easy way to travel between regions, the dialects spoken in California and Maine would probably have diverged farther than they already have.
It's the same as talking with a friend. You two have personal jokes, and there are extra meanings to certain words that both of you understand. Someone across the country from you won't say the same things or express the same ideas with the same vocabulary. People incorporate the words and vocabulary of other people around them, and so the farther away we are from other groups, the more divided our language will be.