How so? Are adventures not largely comprised of the same basic components, the most common and fundamental being the combat encounter?
No? I mean honestly no. They really aren't. Someone else's random ideas about what makes a good combat encounter in the context of a specific adventure are not something I've found much value in stealing in any edition, having played and nicked stuff from adventures since 1989.
What I tended to steal was three things:
1) Maps. So many stolen maps.
2) Ideas/characters/concepts. Sometimes an adventure is based around some tremendous idea but the actual execution is lousy, sometimes there's a wonderful character who would work better elsewhere.
3) Treasures, puzzles, and traps. Specific magic items, and specific traps, can be very steal-able. D&D consistently provides really well for monsters, and building encounters in every edition except 3.XE has been pretty easy, so they're not a problem. But a good trap? An interesting trap? That's gold. So is an actually-good puzzle, though they are vanishingly rare even in published adventures.
Still, a good craftsman doesn't limit themselves by what they have on hand. The "right tool" might make a job easier in an instance. But it is the person who exhibits ingenuity, and innovates despite the inconvenience of greater effort, that improves their skill set. The ability to weave disjointed and otherwise unconnected components into a seamless and enjoyable narrative is a skill best learned through practice. No one ever said GMing was easy or effortless.
Thanks for teaching this grandma to suck eggs, dude!
My point is, the less appropriate material there is, the less reason you want to spend a lot of money buying a big complicated adventure from WotC or whoever. I usually write homebrew adventures so I don't typically need pre-gen ones. But if I want a pre-gen on, I want I don't have to spend just as much effort reading and then fixing as I would have on writing a pre-gen.
And this is the key problem I've seen over and over. When you get a pre-gen, to run it well you need to:
1) Read it through.
2) Understand it - which often requires taking quite a lot of notes, because a lot of pre-gens are really badly written, and really badly organised. Vital description or information that gives a scene its entire context can be a dozen pages away (often later!) in a text box, or worse, in the middle of a paragraph about something else!
3) Make any changes to make to make it "not suck". Hopefully there is no 3. A lot of smaller adventures, there is no 3. They're just solid little adventures, and you do little or nothing to make them work (maybe change a town name or something, or otherwise recontextualize them). With big adventures? There's almost never not a 3, and sometimes 1+2+3 here is significantly more than writing your adventure, often for a lesser outcome.
Does that make sense?