Most people in roleplaying get their start through modules and generated content for good reason. Just now, there isn't any other way to start. The module format is so pervasive that finding a world that does not use some sort of pregenerated content is vitually impossible - and new DMs follow in the ways of the DMs they've had the chance to play under.
it's interesting that, having noted the above, the greatest mass of prefabricated ware for RPG's is oldware. Five years, ten years ... even thirty-plus years old, still used in whatever rehashed form the DM must. The Keep on the Borderland first came to my attention when I was in high school, which was a very long time ago, I can assure you. It gets instant recognition in the Community. It couldn't get more accolades heaped upon it if it were written by Dickens.
Forty years of prefab oldware creates one hell of a populist sentiment, to use Beedo's words. The "populism" accumulates as pressure on new DMs to fall in with the established format: begin by running a module, and someday you'll get good enough as a DM that you'll be ready to ... adapt modules for future runnings.
What else can an established DM tell a neophyte? World creation and sandbox gaming is not something you fall out of bed at 15 and just start doing ... well, it isn't something most players start doing. Particularly since most of them will never SEE it being done. Most will have their first experiences with a DM firmly clutching prefab oldware in hand, reading from (or having memorized) the text, extolling the virtues of what a GREAT module this is, and probably the GREATNESS of the store from which is was purchased, and perhaps the GREATNESS of the designers. Young players are quite easily impressed with such things. Young players take such pronouncements as proof of fact when later on they 'graduate' from Player to DM, getting their own copies of the oldware and waving it under the noses of noobies like they themselves used to be "... and the spirit of time is transferred across the universe to a new defender, and the power of evil is contained for another generation ... and a new Tarakian is born to protect the next."
Or some such justification.
As such, the new players never have any reason to question the books as they are handed down from on high. I'd guess that a considerable number of those defending the prefab method do so from a perspective of one part of logic mixed with twelve to fourteen parts of teenage nostalgia. THEY played "First Quest" with their very first adventure ever, and cue the teardrops staining their cheeks and cue the quivering lips as they remember that shining moment that enlightened their formerly dreary lives. "Don't take our memories away!" screams the subtext, followed by an insistence that they can buy whatever they want because this is a free country.
It is free. And in this free country people contribute all kinds of mediocrity to the general malaise that permeates the dull, mouldering tapestry that universal expression provides. I don't buck the trend. There's little point in trying to teach a pig how to waltz. You get nowhere and it annoys the hell out of the pig. All I want to be able to do is stand next to the couple dozen people (if there are that many on the internet) while I point and say, "look at that fucking bunch of pigs over there." I don't expect any of them to learn anything.
I was lucky. MY first experience with the game when the prefab crap was just coming into existence, and I played my first five years of this game with people who had tried (successfully or unsuccessfully) to run their own campaigns. MY first experience with D&D was with someone who had made his world from scratch, who eschewed modules, and who spat on corporatism ... until he reached middle age, that is, and ultimately did very well as the vice-president of West Coast distribution of Magic, the Gathering. At least, that's what he told me he was doing when I last saw him, which was sometime in the age before the common Internet ... and he could have been lying out his ass. I can't find the guy's name associated with the game anywhere on the net now.
I didn't play a module in any game until I did so in a Traveller campaign ("Prison Planet") a fellow was running, and it took me about an hour to find a loophole that ended with me strafing the planet in a stolen navy ship screaming into the com-link, "I'M A MODEL PRISONER!" It was kind of fun. It was mostly fun making the ref of that game look like a bleeding idiot, which we did quite a lot. Poor Irwin. He never could take anyone's advice where it came to running prefab shit or his own.
Again, I digress.
I suppose that not finding modules very interesting to play in (when they were obvious railroads or death traps), or rather easy to get on the other side of (when they were loose and role-play oriented), did eventually combine with a changing universe in the 1980s when more and more new players in the game rushed out to the store to buy their drugs before sitting down with a piece of paper and a brain. True, many of those people would have run a much worse campaign than they could buy prefab; but buying prefab guaranteed they'd never run anything better.
Thinking about the nostalgia of youth and a person's first prefab experience, and the nostalgia that entails, let me point out that a long, long time ago I sucked on a tit once, too. I won't argue that I don't have a certain nostalgia for the owner of that tit. Still, I'm not much interested in sucking my mamma's tit now.
That whole weaning off phase - the one that takes some folks way too many years, when you start seeing your mother as a person and not the facilitator of all your dreams, can take awhile. It's often loaded with moments of embarrassment, and failure, and dispair. The world is mighty unforgiving of everyone's inadequacies, and it sure is easier to rush back home to that tit when things get hard (heh heh).
Mamma WOTC understands. Mamma kiss it better. Mamma'll get out that big ol' tit and give babykins what he needs. And don't forget, babykins ... Mamma knows best.