I suspect it has to do with control. As a writer, there's a lot you can't control: a lit journal accepting/rejecting your essay or short story, an agent requesting/rejecting your full manuscript, a publisher taking on/passing on your novel/memoir, etc. Writing meaningful, rewarding stories and improving as a writer should be the goal, but that's hard to quantify. Writing output, however, is easy to quantify and can provide writers with the feeling that they're making progress. Consistent output is also rewarded in online publications like Medium.