Because the content on Twitter is generated by a relatively small number of users. A lot never tweet, quote tweet or retweet but it goes beyond that number once you weight it by audience. A small number of people have a large amount of reach and thus are responsible for a good chunk of the content.
That content is why the users are on Twitter and it is those users who are advertised to that pay for Twitter to exist. That's what Stephen King means.
Now you can turn this around and say that those power users are only there because the audience is and that they get value for being there but platforms need users and users follow creators more than the platform as a general rule.
