Daniel replies:

This is very easy to answer.  There are two answers:  1. Because I wasn't any good at any other job.  2. Because writing is more fun than any other job.  I will expand Answer 2. to say that I have had many adventures.  I have been all over the world, I have climbed mountains, I have faced wild lions, I have met famous and interesting people.  Writing is more fun than any of those things.