> What is making it complex for authors to sell directly?

If an author living at place X sells directly to a reader living at place Y and it is not true that for all governments G PlaceInJurisdictionOf(X,G) == PlaceInJurisdictionOf(Y,G), then the author is making a cross border sale which can have annoying tax considerations.

The internet made it very easy for someone to physically run a small business that conducts transactions across dozens or even hundreds of borders, especially if they are selling intangible goods, but as far as I know the legal environment has not been updated for that.

Hence it is a lot easier to use a middleman who handles the legal stuff with cross border transactions and you only have to deal with the middleman.