>I belive most people could write a book that a significant number of people they didn't already know would pay to read (this may fall well below the numbers that would make a publisher interested)

What's mostly missing from this discussion is marketing. Sorry, but even pretty good books will languish on Amazon (which is mostly what matters) if you just put a book out there, fiction or non-fiction, with the attitude that "If you build it, they will come." (Like most everything else.)

Unless you're a known quantity for other reasons, even getting accepted by a publisher is a tough slog and there is a long list of stories of publishers (like studios) passing on blockbusters. Not that getting in with a major publisher is any guarantee of success but cultivating a fan base as a total independent self-publishing is that much harder.

(In addition, as someone who does a lot of writing and editing, you're also probably optimistic about the abilities of said random person.)