Image via WikipediaThere are three reasons, actually. The first is my glaring lack of talent, though I can name probably fifty professional authors that haven't let that stop them.
The second is the appalling rate of pay. While I dream of making John Scalzi money, I'd almost certainly make something below Justine Larbalestier money--and that's no knock on Mrs. L. She is teh awesome, but clearly under-appreciated and underfunded. As are most novelists. That's why most writers have day jobs.
But the main reason Im not a professional novelist is that, to do what Charles Stross does, I'd have to average an output of 1000 words per day. And that's finished work, mind you, discounting edits, rewrites, and those days when you merely manage to vomit something onto the page which deserves nothing less that complete and total abandonment, if not outright exorcism from the memory of the universe. Oh, and if one were to get sick or take a vacation, those thousand finished words would needs be made up on another day, skewing the workday average.
I'm not sure I speak 1000 words per day--certainly not 1000 usefully repeatable words--let alone could average such an output in written form. This is why I do about 6 billion other things besides write: partly because I'm better at them than writing; partly because they pay more. But mostly because it's an output I can sustain.
More's the pity.