That's ambiguous, isn't it. Why not, write! That comma indicates that you should just write, dammit. For no reason or for a few reasons, or even for a myriad of reasons. For example, you have something important to say, people need to hear it, you wish to educate folks about on an important subject, you want to entertain, experiment, express yourself, be creative when you're tone deaf, forget your lines, and have no rhythm, feel you have some talent, Me, I love TMR's, like libraries, like the feel of books, like reading them, feel good when I'm writing, love creating something from a blank page. want fame, glory, and riches. Even when I hold a book I've created, or a journal with a story or an article with my name on it, whether fiction, or non fiction, I feel I've achieved something, I just feel bloody good and want to do it again. Bottom line? I think some of us are just wired to write. Just like some of us are wired to play the bassoon.
Or another way to read it is Why, not write! That's asking why anyone would want to write, that is to say, identify themselves as a writer. I saw the biopic of Toni Morrison tonight. She worked as an editor for Random House, taught writing at Princeton, She said she never called herself a writer even after publishing a bunch of novels. Said she was a teacher who wrote, or an editor who wrote. Then when her boss recommended she quit her job as editor in order to write full time, she finally had to acknowledge that she was first and foremost a writer.
The world is full of people who aren't writers, that is, until you meet them at a party when they have a few drinks in them. A recent host of mine pulled off the shelf a couple of books that he'd written. One was a slim paperback of jogging routes around Jerusalem now made moot by Trump's Deal of the Century making jogging in some neighbourhoods unsafe. His other book was an illustrated book about a girl who hated her curly hair, just like my host's granddaughter. These proud offerings took precedence for the professor who'd authored a ton of articles that advanced the state of the art of his medical specialty.
There are reasons not to write, most of them familiar to one who hasn't done one's homework. Too hard, and the corollary--tried it, and it's no good. Want to do other stuff (Apps), Have to do other stuff (Chores). Too tired. Don't know how. Don't want to. No time, And the Killer--Nobody Will Read It. Most real writers I've heard of, feel this way at times, and some use antidotes (Cheever, Fitzgerald--alcohol. Updike--golf).