Honestly? I think this is the direction that the producers of D&D have taken to take the workload (or funload

) off the DM's shoulders for the most part. The VAST library of adventures for D&D means that there is pretty much someone for everyone. And, once you do use an AP (I'm running Ghosts of Saltmarsh myself), so much of the yeoman's work is done for you. It does make running a campaign a lot easier.
The massive popularity of the 5e adventure path's, IMO, speak to this. It's never been easier to sit down and run D&D. I mean, good grief, how many hundreds of hours of play could you get out of the 5e AP's right now without a whole lot of work needed from the DM.
Using prepared material is certainly one of the ways to save prep time. However, you generally still have to read through the material, and those big books can take a lot of time to read. I ran Tomb of Annihilation about a year ago as part of my 5E campaign, and I found it difficult for a number of reasons. I've since decided to only run my own material, and I am finding it far more easy.
But, I've also been DMing for a long time. I think for those starting out, using prepared material is a good idea. Although even that won't apply to all (I think
@prabe mentioned that he can't make sense of the APs). I think that smaller bite size chunks are far more helpful to newer DMs. A few pages and a map, and not a whole lot more. I think the Adventurer's League stuff may be more in line with what's useful to a new DM, and certainly smaller adventures that can be found on the DMs Guild.
I'm honestly not sure it does, though.
It changes GM prep requirements, no question there, but the effort level is similar*. Instead of working on designing the adventure etc. from scratch, the effort now goes in to tying the PCs' backstories together into something coherent that still fits with the setting.
* - remember, you're used to doing it this way, so what's limited-effort for you might not be to someone else. Conversely, I'm used to designing adventures from scratch and so find it less effort than would someone who's not done much of it.
An analogy might be the use of published modules. For some GMs they cut the prep time down to nearly 0 - just open the book and go. But for others they don't cut the prep time by much at all, they just change the manner of work that needs to be done: instead of mapping and doing up stat blocks while having the backstory already done for you as part of your campaign, you're instead stripping out backstory and editing stuff to make it fit in your campaign while having the mapping and stat blocks already done.
I agree on published modules, as I mentioned above, but I think the smaller the better for newer DMs, generally.
But when it comes to getting PCs' backstories to "fit the setting", I think it's useful to kind of look at it another way. What if the setting fit the PCs' backstories? Sure, you may have a general setting idea such as "we're going to play Dark Sun at the time of the fall of Kalak" as
@pemerton used as an example. But what if that's step 1, step 2 is the players make their characters and related concepts (whether backstory, related NPCs, kickers, or some mix of all), and then step 3 is that the DM fleshes out the other details?
This allows the player suggested material.....material they'll likely have more vested interest with....to be prominent. Then the DM can structure the missing pieces around that.
Certainly this will reduce the DM's need for prep at least as much as what the players bring to the table.....maybe some NPCs, factions, locations, or connections to existing ones. It also gives the DM very clear cues about what the players would like for play to be about, which can help when it comes time to mine for ideas for conflict or adventures. Many of the "adventures" in my 5E game largely write themselves because the players have an agenda other than the potential agendas I sprinkle into play.
I agree that this stuff can take time to get comfortable with. But the earlier people begin, the earlier they'll get the hang of it.
There's some other things one can do also, but they don't so much reduce overall work as move a lot of it to a more convenient time - that being, before play ever begins.
My favourite among such things these days is to, long before session 0, have your setting's history (ancient and recent), pantheons, and local geography pretty much nailed down. That way, once you start running the game you can in effect riff off yourself, in full confidence that everything will remain consistent. The best part once play begins: the hard work is already done!
Similar to Tolkein largely nailing down Middle Earth's history etc. before ever putting his authoring pen to paper.
Of course, much depends on how much mileage one intends to get out of a setting. If you're looking at running a 10-month single-linear-party quick-hitter of a campaign you can 99.9% likely get away with tons less setting prep than if you're looking to run a 10-year multi-party behemoth.
Two things on this. I think that using a pre-established setting can indeed cut out a lot of work on the part of the GM. It may also serve as a hook for the players because they may already have an understanding of the setting and possibly strong interest in it, too. This is a big part of why my 5E campaign takes place in the D&D multiverse. It allows us to use any and all published bits, and a lot of the big players are known NPCs that the players already have an interest in.....Eclavdra and Iggwilv and Snurre and Mephistopheles and Graz'zt and Shemeska and so on. All of this gives me a plethora of material that I can mine for ideas, and the same for my players.
Having said that, I think that telling a new GM that they need to construct an entire world with all these details predetermined is one of the things that perpetuates the idea that it's so difficult to GM. There's no reason that this stuff can't be built as it's needed. Start with a town or city and the surrounding region. Add members of the pantheon as needed. And so on.
There's nothing wrong with a Dm who's experienced and has a clear idea of a setting or world that he wants to craft for his group. But I don't think it's good advice for new GMs. It's too daunting. And the chance that they'll do it all so well that they'll actually get to use it all? Seems pretty slim.