You say "accents", so I take it that you mean people who speak the same language?
Isn't it even more remarkable that so many languages have "come about", to use your phrase?
We learn to speak first by copying our family; as we grow older we copy our friends as well. We do this because we want to fit in, to be part of the group etc. If we move from one end of the country to another, we often adapt again; if we watch television we pick up phrases and intonations from other countries; and so it goes on. Listen to old recordings of people who are still alive and compare this to the way they speak now; often there is a quite surprising difference in accent.
What does all this mean? Language is a living, evolving thing. It is fluid and flexible. A fellow like Geoffrey Chaucer would probably find it hard to make himself understood in Canterbury in 2011, yet he spoke English as far as he was concerned. Shakespeare would converse with your average Brummie with a lot more ease. Perhaps he'd have found it easier to chat to Chaucer than you or I would. He certainly would have thought it strange that we put so much emphasis on spelling words uniformly - for many centuries a phonetic representation of each word was considered adequate, even though it seems terribly confusing to us.
Like other things that evolve, division and isolation of a language will result in big changes. Look how differently Americans now speak compared to the British; and how different was the English of the Pitcairn islanders compared to modern British English.
Richard Dawkins coined the word "memes" - you might find it interesting to read about them.
