By ‘unpredictableness’ I assume you mean that we rarely feel totally inspired every single day. I sure as heck don’t. But I do it anyway.
First and foremost because I write because I like it and it is good for me. Second, I know that if I wrote daily, I stay in touch with the story and I write better as a result. Third, the writing pays my bills.
Any career in the arts has a simple truth attached to it: you have to do the work every day. That is how you get better.
It doesn’t matter how many books I’ve published. I have never before written the book I am writing now. I have to respect the work and keep striving to learn more, keep searching for new tools for my work chest.
If you are standing on the outside looking in, it might seem a little boring, and I admit, there are days when I long for a job that has a guaranteed paycheck every two weeks and some kind of health insurance, but the truth is, I feel incredibly blessed to be able to write stories that people want to read. That is extremely motivating.