You are right and that's the way industry is moving slowly. Currently publishers provide the following:
- Toolkits and tutorials for inexperienced writers. If you never written a book, you probably don't know first thing you should write is outline. You probably also don't have good templates with typography and layouts that gets instantly recognized as good modern book.
- Taskmaster that keeps nudging you: This is super important role. Book writing is tremendously taxing job for most people and its very easy to get in writer's block, put off things and distracted.
- Regular advances: Almost all publishers would deposit money in your bank account at specific intervals during book writing. This keeps you obligated, at least morally, to keep going.
- Book reviews, editing: Various services like cover design, proof reading, recruiting technical reviewers etc.
- Marketing: Almost all publishers would buy sponsored spots on Amazon as well as Google. Some top-tier publishers would have journalists do book reviews, podcasters invite you for interviews. Some very top-tier publishers would have you do book tours at their expense. However, 90% of the authors will end up doing virtually all promotions themselves.
- Brand recognition: Publishing from very top-tier organizations like MIT Press has huge brand recognition due to their stringent selection process.
I think the future of writing is not even book-size publishing but probably series of posts published as chapters online.