Everyone is part Introvert and part Extrovert. I'm definitely more Introvert than most but again I can quite easily give public speeches if and when I have to. I also dress very extroverted which many would say is in conflict with my personality.
I also have a friend who is a very gifted writer and she is extremely extroverted. I agree with ithabise that whichever 'vert' you are more is more about how you gain energy~ so possibly the fact that writing is mostly a solo adventure makes it an ideal path for the more introverted who tend to be deep thinkers rather than sociable talkers.
Now public speaking is more to do with shyness which is a completely different subject as you can be shy whether you're more introverted or extroverted.
'The Loner's Manifesto' is a fantastic book which goes through all these differences (again loner's, outcasts and introverts and all seperate types), everyone should read it tbh :)
Good question btw xx