Why? You don't need it open to you, it didn't exist two months ago, so why do you require it to be open to you? Yes, it is your job as the DM to challenge the PCs. No, it is not your job to turn every single thing into avenue of possible challenge for the PCs. Those two things are different.
We disagree there; to me those two things are part and parcel of one thing.
Why? First of all, there is not a single divination spell in the current game that can do this, except maybe Detect Thoughts, and most people don't get access to Detect Thoughts. So... trust no one unless you have read their mind to know they can be trusted?
Zone of Truth is still a thing, is it not?
Failing that, there's real-world options such as references, getting to know the person, etc.
I did retail management for ages, and part of my job was interviews and hiring. I'm not bad at reading people in person but over the years I still got burned a couple of times: one of my hires started stealing from us pretty much on the first day of work and another got us all in real hot water with legal over some interactions with a customer (a very long story). But that's just two out of probably 50-60 people, so not a bad batting average.
That's the reality. Which means that's what I'm going to put in my games: that there could always be that one bad apple.
Same as traps - not every door in a dungeon is trapped but some of them might be.
And you know, the worst part about this? This is exactly why players become murder-hobos. It is a classic negative reinforcement. You won't play a character who will give out charity if your charity is usually followed by being betrayed and hurt. You won't play a character who will give someone a second-chance because most of the time they do it, they end up being made to regret it. Players can't have property and places to go home to without someone else to maintain the property. And you can't have someone to maintain the property until you can equip them with enchanted shackles, mind read them monthly for thoughts of betrayal, and have them sign magically enforceable contracts... because you know, you can't just blindly trust people.
Or you just see how it goes. The servant my character has at her house in the game I play in has shown me more loyalty than any other person I've ever met...and taking him on was a pure leap of faith on my part; he's a Goblin (they're monsters, in our games) who was servant to some bad guys we knocked off. We didn't know what to do with him so my character took him in and hired him, not quite knowing what to expect; and it turned out to be one of the best moves that character has ever made.
But it took me a while to realize just how loyal and trustworthy he was. Until then I took precautions, not knowing if he was going to slit my throat while I slept, but soon realized I could relax on that.
Even if you don't do it "all the time" in real life it only takes a single major betrayal to wreck someone for life. IF 50% of your NPCs are just poison pills? Then the players stop trying to invest in them. Because it isn't worth it.
50% would be overkill, I agree.
But 5%? That's more like reality, IME.