Upon re-reading your post, there are a few misunderstandings I think about how Amazon publishing works (either that, or I misunderstand them). Yes, you'd need to pay for your own advertising, if you want it. If you don't want it however, and just want to put it up tagged so it can be found, then that's an option too. The Amazon rate to "buy copies" of your own book will almost always be much cheaper than going to a printing shop, due to economies of scale. It has it's own requirements (size, format, quality), which a printing shop wouldn't, but for a little more effort, the book could then be purchased by anyone.
The big reason you want a publisher imo is to handle all the tasks of finding quality people. Finding an editor, finding an artist, finding a formatter, finding a market analyst, etc. So, if you're just publishing for yourself and a few friends, there isn't really a reason to go through a publishing house. It entirely depends on how valuable you think your own time is as a writer, and how good you think you are at finding people to do those things (or doing them yourself).
No matter how good of an online author you believe you are, the peripheral tasks related to selling a book are more important. If you're super famous like King or Sanderson, then you don't need marketing anymore really, but if you aren't them, then a publisher offers a path where you can keep writing while the publisher does all the non-writing work around actually making your story publishable.
Someone already made a thread a few days ago with a complaint about being unable to easily edit their manuscript for something they've published. Thus is a reality for books once published.