After careful observation of the phenomenon, I'm pretty sure that what's going on is that publishers have developed a canny method for distinguishing bad books that make them money from bad books that lose lots of money.
I like to think that we can tell. I mean, there is the guiding principle that we also have to like the book, which wins out far more than you think. Many, many times I've said to various bosses, "Well, it's the kind of book that isn't very good, but will probably sell a million copies" and then they've said to reject it. When agents get behind bad projects, it generally isn't intentional. But if it's bad AND it won't make money - that we can smell from a mile away.
"I've discovered the secret to the universe and it's this weird idea about body energy!"
"You can lose weight if you cut back on your calories and exercise."
"My grandfather was full of old man wisdom and I'd like to spend a book talking about it."
"This gripping epic of a Civil War-era family spans the final decades of the 19th century and ends with the First World War."
"I wrote this book with the help of G-d. No, I mean literally."
"The real gospel was hidden away by the Templars for some reason I never properly establish, but in it contains the true message of Christianity, which is that the church sucks."
"The story of my cat's battle with cancer will be an inspiration to millions!"
...Yeah. Sometimes my job isn't very hard.