But,
is dead on target. I'm not reading the Monster Manual for enjoyment. I'm reading it because I need a critter to eat my PC's. I don't know about anyone else, but, I generally start writing the adventure first, and then populate that adventure with critters. "Hey, there's a hole over here, let's chuck an Otyugh in here. Oh, wizard's storage room, what kind of weird goodies can I have crawl out of that box?
I've never gone the other way - "Hrm, this bit of backstory about this monster is really interesting, let's write an entire adventure around this". I've certainly taken novels and short stories that I've read and turned them into adventures. But, the Monster Manual has almost never led to anything in play.
Do people just not read very much?
I don't think the MM is really aimed at experienced game-masters, to be honest. I don't even use the stat blocks and tend to conclude that I just spend $50 on an art-book. I suspect
someone is probably inspired by the flavor of the monsters in the book. I buy the book for the art, you buy it for the stats, I'm sure someone buys it for the fluff. If nothing else, the little fluff block can provide a new DM unfamiliar with the history of the creature or D&D, some guidelines on how to situate a Dragon Turtle in their games. And for people with lower-levels of inspired ideas floating around in their heads (again due to their newness to the material) it may provide some exciting inspiration.
And to your final question: no, I think a lot of people in this day and age expect their information to be very sound-bytey. Compressed into relevant chunks of information that can be quickly assimilated.
But then, I don't really think the
fluff that comes along in the Monster Manual is quite the same as the setting lore that comes in a Campaign Book. For example: when I run Ravenloft, I care nothing for the mechanics of the monsters, what is entirely important is getting the right feel and style to the campaign, which is 100% from those fluff nuggets. And in that context, I most certainly have used elements of the fluff as jumping-off points to spin new adventures within the setting (and without) and alter certain elements of the game (such as removing the Raven-kin and revamping their related religion to one based around the Raven Queen).
As was also mentioned, some folks enjoy being authorities on setting lore (see: Star Trek or Star Wars fans and
those setting lore books). And I think some players also expect the DM to be a setting authority. The latter is a place I always struggle with in campaigns because I like to leave my homebrew worlds fairly open until its absolutely necessary to have specific information about parts of them, but there's always
that guy....
Also in your mentioning of the Dragon Turtle I thought it would be interesting to have Dragon Turtles based on different types of real turtles, which then took my mind to silly places (not Camelot but NYC) and I thought up Teenage Mutant Ninja Dragon Turtles. And the fact that Pathfinder has both a Ninja class and a Kappa race, and I have an upcoming Pathfinder game, I thought it ought to fun to include some little side-adventure related to TMN Dragon Turtles. No, the fluff in the book had nothing to do with this. But again, I doubt you or I are really the target for the little block of fluff in the book.