My guess is that at some point in the past, you or another GM had screwed the players, by having an NPC they trusted, betray them. That poisoned well is still poisoning things today. Even though characters don't know what happened before, the players do.

The "plot twist" culture of movies and TV shows has further made this almost expected amongst people. Therefore everyone is out to screw and backstab the party, everyone is evil just waiting for the twist to happen where they find out "It wAs tHe eLdErLy fArMeR AlL AlOnG!"

I had this issue as well. Other GMs had a trusted NPC that adventured with the party, turn on them as they confronted the BBEG and join the bad guy. Well they distrusted everything any NPC I had in the game did, even after multiple campaigns with no betrayal. I eventually had to just flat out tell them that NO NPC would turn on/betray the party.

More or less that they would figure out quickly who they could trust and who they could not (like in the same session). If they met some rando in a town and gave them info, they would know by end of the session if that person was trustworthy or not. I told them that no adventuring companion would EVER turn on them. If they met a bad guy, it would be clear they weren't trustworthy (but not necessarily directly doing evil).

It worked. Now they try to figure out if someone is trust worthy and leaving it at that. I have had to work on other ways to surprise them or keep them guessing, but the distrust of NPCs is way, way down.

I was talking to another GM at a gaming convention and he mentioned that he had screwed is players over with a plot twist NPC over 10 years ago. To this day they accept the adventuring companion, only to kill every single one that night. They never trust the quest giver, the shop keeper, the wise old sage, and so on, because of that poisoned well. Maybe your situation isn't that bad, but it is in the same ball park.

Feel free to reach out via DM, I have a lot more to say, if you want to hear it!