The author is the one doing the one-time effort. After the author finishes, he turns it over to the publisher, who takes the one-time writing effort, adds to it

The author is the one doing the one-time effort. After the author finishes, he turns it over to the publisher, who takes the one-time writing effort, adds to it editing, layout, graphics, printing, reprinting, inventory management, etc., creates a product out of a manuscript, then has their sales force sell that product on an ongoing basis from then on. While the publisher's work never ends, the author's ongoing task amounts to cashing royalty checks and complaining. Or that's how publishers see it. Having said this, I should add that I'm both an author and a publisher, meaning I self-publish. I think traditional publishers take so outrageously much (in money and control) and deliver so ridiculously little that I'd never let them turn my writing into their product unless I were a celebrity.