A lot of authors are not technical people and/or are busy writing. There isn’t a simple system with a large audience of potential book purchasers that makes it easy for authors to sell their books because Amazon became that place but now wants to irritate its users by mangling the kindle.

> 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.

Also unfortunately the problem lies when the platform has too much power and makes this relationship with the author disproportionate. At first such a platform seems like a good deal for everyone, except when it stops being one