I used to run out of inventory space on my D&D character sheets. I was utterly fascinated with packing just the right tool for every sort of situation, and I spent an unreasonable amount of time figuring out the lightest, most useful kit I could pack. As a player, it's a lot of fun to come into a situation and have just the right tool to short-circuit the challenge and move on (especially because the challenge is almost certainly unfair in a substantial and gygaxian way). There's sort of a double satisfaction to this because, outside of fiction, it's what good problem solving looks like: finding the easiest, most effective solution with the tools on hand.
Unfortunately, that can make for a very boring game (and a frustrated GM).
A lot of adventure design gets committed to keeping things from being simple. The logic behind this is reasonable enough: simple challenges are quickly resolved with little sense of risk or engagement, and that is a recipe for boring play. Unfortunately, the obvious solution (making things arbitrarily more challenging) is workable but ultimately counterproductive.
To illustrate this, consider a dungeon. There might be some reason to go into the dungeon (rescue the hostage, let's say), but how many of the challenges you're going to run into have any bearing on that? In some adventures, they might all be, but I think we all have experience with the adventure where there's a mandatory quota of fight scenes with random-seeming monsters. Those encounters "flesh out" the adventure and keep it from being too simple.
But that's sloppy design. It's LAZY. Look at it this way: if I have a bunch of cultists take a prisoner, there are lots of ways I can make the adventure more challenging. I can include an important NPC among the cultists backers, meaning I may face legal barriers keeping me from pursuing the cultists. I may face hard choices in terms of the price of stopping them. Maybe the cultists are not so morally black as I think, calling into question the righteousness of violence as a solution. I can introduce a second challenge (Burn Notice Style) and make the real difficulty in juggling both concerns.
Or I can just add a few more monsters/fight scenes.
Now, don't get me wrong. I love good fight scenes. If I want to add more of them, there are plenty of ways to do it that make more sense then "And behind this door lives a shambling mound!" Similarly, I'm sympathetic that for any published adventure, the lack of hooks into the specifics of the campaign being played limits options, but I must add that it doesn't remove them entirely. There are too many examples of good adventures to pretend it can't be done.
The specific solutions for this are going to depend a lot on your game, but the question it raises is always going to be the same. When looking at a challenge you're going to throw at your players, ask yourself how it's going to make the game better as well as harder. There are lots of good answers, including "It will be an awesomely fun fight", "I need to give this player some spotlight time" or even "Holy god, I need to fill an hour - Fight time!". Just make sure you have an answer.