> e.g., we handle our own formatting

I used to work for a small publisher some years ago, and while this is true to some degree, we spent a lot of effort doing additional formatting or correcting formatting mistakes. For a typical journal publication, this process alone takes weeks if you're aiming at a high-quality publication.

On top of that, there are a lot of small things that you typically don't get if a paper is just put on the author's website, such as e.g. long-term archiving, a DOI, integration with services like dblp, metadata curation, etc.

Now, to what degree these features are an added value to you personally varies from person to person. Some people or even workshops are totally fine with simply publishing the PDFs written by the authors on a website, and there's nothing wrong with that, ymmv.