I don't think it's a matter of the cost of self publishing. It's the benefit of having someone whose full-time job is to distribute and promote your book. Having every single author become an expert in that is hugely inefficient, and it's probably not what your average writer wants to be doing.

Just because you can do your own dental work doesn't suddenly make it a good idea, much less an economic one. For work that's somewhat specialized and very intermittent in nature (like promoting a new book) it's often better to aggregate the labor.