This argument is akin to saying "now that anyone can publish a blog or tweet, what's the point of news places like NYT or WSJ?" I don't believe either negates the needs or benefits of the other. In addition to what others have mentioned, with publishers (just like with established news organizations), you can (usually) expect a certain level of standards and thoroughness with the end product, at the cost of fewer and less diverse works. With self-publishing, you can find end products that might have been passed over by publishers for various reasons (not a big enough market, too controversial, etc). Each is beneficial, each is necessary.

Publishers pay authors up front to write novels. They also provide quality editors and marketing on their dime.

So, yes, there is absolutely value in what the publishers provide.

Author here.

At least, from my experience, they are. No Starch Press went above and beyond when working on my book, and they're currently getting hit hard by the counterfeiting issue.

Not like there's a need to justify anything here, but I'll go over some of what a good publisher does. Multiple waves of editing with an editor who knows you, knows your content, and works with you every step until everybody is happy. Technical review, where they seek experts who ensure the validity and accuracy of the content; just like we developers use code review, good publishers provide tech review. Copy-editing, proofreading, redrawing of graphics, constructive feedback, the list goes on. For an author, all of these steps are quite daunting, but, by then end of the process, the content is far beyond what it was upon initial regurgitation. That's not even considering the amount of marketing they do after release; getting authors radio and podcast interviews, news articles, and book signings at conferences.

Have fun soloing that and still keeping your day job.

They're adding a lot of value. Part of my novel-buying process on Amazon is checking the publisher, specifically because self-published books have such awful writing.

You didn't like "The Martian", did you?

No, I didn't, but in that case the problem wasn't the horrible writing I complain about above. "The Martian" has much better writing than the general run of self-published novels.

Some publishers do, some publishers don't.