Not OP, but for me, kind family and friends, and various feel-good pieces of fiction and other writing, at least let me envision the possibility of a perfectly kind/dedicated/innocent/naieve individual who is truly on my side 100%. But even that is mostly imagination and fiction... although convincing others of that isn't necessairly an argument worth making.
Commercial entities have a fundamental purpouse of profit. While profit doesn't have to be a zero-sum game - ideally, everyone benefits in a somewhat balanced way - there's some fundamental tension, in that each party's profit is necessairly limited by the other party's.
Government entities have a fundamental purpouse of executing the will of the state, which is rather explicitly not the same thing as the will of you as an individual.
Both commercial and government entities also tend to involve multiple people, which gets statistics working against you - you really gathered that many people who would put your needs above their own, with exactly zero "imposters" - which in this context just means people with a bit of rational self interest?
> I guess I'm trying to wonder why this line of thinking (in theory) doesn't turn to paranoia about everybody. I don't know much ethics or political theory or anything.
Just because you're paranoid, doesn't mean they aren't out to get you. Trust, but verify.
You might not be able to put absolute blind trust in anybody. I certainly can't. However, one can hedge one's bets, and diversify trust. Build social circles of people with good character, good judgement, and calm temperments - and statistics will start working for you. It's unlikely they'll all conspire to betray you simultaniously, especially if you've ensured betrayal costs much and gains little. While petty and jealous people can indeed be irrational enough to betray under such circumstances, it'll be harder for them to create the kind of conspiracy necessary for mass betrayal that might cause significant enough damage to warrant proper paranoia. You might still have to watch out for gaslighters stealing credit (document your work!) and framing people (document your character!) and other such dishonest and manipulative behavior... but if everyone's looking out for the same thing, well, that's just everyone looking out for everyone else! That's a community looking out for each other, and holding everyone honest and accountable. Most find comfort in that, rather than the stress paranoia implies.
Put yourself in a room full of manipulators and schemers, on the other hand, and "parnoia about everyone" might be the only reasonable or rational response!