Besides the distraction of the Internet, once a writer is published there are other things that get in the way of actual 'work'. It should be a writer's job to just write. That's rarely the case, however. Once a book is out there is so much to do, no matter who published the novel that time gets stretched so thin you wonder if you'll ever be able to actually just write again.
Marketing, and promotion take up hours upon hours of valuable time that could have been spent on your next manuscript. Necessary evils that consume a writer's mind once that beloved child has seen print. If no one knows about it, no one will read it. Publishers often take out print ads in big name journals and papers, and make sure your book gets into the hands of reviewers. The reviewers can help, but I really have my doubts about the print ads except for the little bit of euphoria it gives the writer to see them, and giving the ads a small nod for their name imprinting, unless your name is one of the mega-stars it probably won't mean too much in the sales of any particular book.
Hands on marketing in the form of web-surfing, bookstore cruising, public appearances do much more, in my opinion to sell books than the above mentioned means. Those efforts take up huge amounts of time, however.
Why can't a writer, just write?