Here's how Colum McCann answered that itch-inducing question in an interview on the Powell's Books Blog:
"Thomas Berger has the most beautiful answer to this question. 'Why do writers write? Because it isn't there.' Enough said. Or almost enough said. Vallejo said that mystery joins us together. And Joyce said that he wanted to create life out of life."
Yes. Because. It. Isn't. There.
I'd never heard that Berger quote before.
And here's another question/answer from the McCann interview I found interesting, given my appreciation of Don DeLillo:
"If you could have been someone else, who would that be and why?
"I teach at Hunter College in New York and recently had Don DeLillo come to class. It was an extraordinary day. He was incredibly profound and moving and gracious and just plain honest with the students. I was also stunned by his humility. At one stage he said to us, 'I seem to be the beneficiary of an occasional revelation.' This is the man who wrote Underworld, one of the best novels of the last 25 years. We went out afterwards with a couple of students and had dinner, and a few drinks, and I watched him climb into a cab, and I thought that I would like to be that mind, I would like to sit inside that mind, if even just for a while, traveling home to Bronxville on a March night in 2009. I would very much like that indeed, to be going in that direction."