I think the key thing here is that the vast majority of groups aren't going to see any problem with things like the Hill Giant guards. My players would just say "Alright, we need to get through these doors. There are Hill Giants guarding them. How do we get past them?" without every wondering WHY they are there.
Maybe one or two would concern themselves with the WHY and that would likely go: "Hmm, I'm sure there's a reason these are Hill Giants here. We don't know it and we will likely never know unless we interrogate everyone in this complex and that would take way too much time so that reason really doesn't matter."
To me, as a DM, the reasons don't really matter either. The only thing that matters during the game is the description of the guards and the players finding a way past them.
I can think of a hundred different reasons why the guards are there. All of them are logical but not all of them are OBVIOUS. I think that's the key here. Most of the time when I see people complain that something isn't logical it's that they expect the absolutely most obvious answer to be the correct one and anything not completely obvious to be "reaching" or "coming up with excuses why something isn't logical".
I was recently involved in a discussion about "My Guy Syndrome"...the idea that "My Guy would do this! It's the only thing that makes sense! Sorry, I know this will ruin the game, but I have no choice, that's what My Guy would do!" I believe this is just an extension of that. A large number of people tried to argue that there is space between what you want and what your character wants and you have the ability to make your character do whatever you want since they are YOUR character, not completely independent of you. If something wouldn't be fun for you or for the other people at the table, you have the ability to come up with reasons why your character decides to act "out of character" for this one time in order to make the game more fun. Or even to slowly change the personality of your character as they grow as a person.
I find the same thing is true with "illogical" encounters in adventures. If it creates a fun scene where you get to negotiate with dumb Hill Giants or with a giant who lives in a tower wearing a Wizard Hat then I can come up with a reason why it happens to make the game more fun.
If the only thing that ever happens in a game is whatever is the most obvious, then it is not only extremely predictable and boring but it can actually create a bad game. What happens if the most logical thing is to place all of the best trained and most powerful giants as the guard to the room? The PCs might not be powerful enough to make it past them to continue the adventure. The final encounter might seem anti-climactic because the "final boss" is less powerful than the people who were guarding the doors.
Often the most logical situation also isn't the best story or the most fun. Given the choice between the two, I'm glad WOTC often picks the fun route.
Ah, the strawman of, 'if you don't embrace the illogical encounter in adventures, you're just doing the obvious and that's not fun.'
Let me give you an example from my game last night: the party almost wipes on the stone giant Thane, but pulls out with one death. They find Ellister, the rock gnome, and I, having read his description, play him off as brash and excitable and a nonstop talker. Through the scene, they party finds out that Ellister is from Loudwater, that he's offering a reward for returning him to Loudwater, and how the Thane was using the stalactite. Ellister also took a shine to the dwaven warlock PC, even rushing to his aid to cure him in a later fight (and ignoring the barbarian, who was far more badly wounded). This was a blast, as the dwarven PC didn't want an excitable gnome talking his ear off constantly, but also didn't push him away. When Ellister saw the airship, he flipped out, and went rushing all over it. He won over the captain of the airship by making useful recommendations for improvements and got greater access. The party was happy to get improvements, and so left the ship with Ellister making changes to go to Waterdeep via teleport circle for a few days to re-equip. When they returned, Ellister has stolen the airship and gone a joyride across the Sword Coast.
This all played out directly from how the party interacted with the character, and with the provided motivations of Ellister, and with opportunity within the game. Totally not fun, right?
But Hill Giant guards on the embattled Storm Giant royal family in an adventure about the ordaning being broken? Nope, should just glaze over that. And you're right, most of the time players don't notice. However, I, as DM, need to present a world that reacts believably to the players, and knowing why there are incongruous hill giants as the royal guards is something that I need to know, especially if the players do attempt to bluff them. Why they're there and what their motivations are is crucial to being able to run that scene believable and well, and even in a fun way. But, pointing out the complete failure to provide an explanation for this very incongruous setup makes me an unfun DM, I guess. I'll let my players know, they'll be disappointed in me.
