That's not always a good thing, the world might have been better off without some of Piers Anthony's work.
I said "prolific", not "good". Though, Asimov is likely my favorite author, and ordinarily I'd be loathe to mention Anthony and Asimov in the same breath. Just goes to show that both good and bad writers can be more successful just by virtue of writing a lot.
This, I suspect, is actually an effect, not a cause. Isaac Asimov and Piers Anthony (both famously prolific writers) have said that they don't get writers block because they never go a day without writing, not the other way around. In other words: the more you write, the more you'll be able to write.