I feel like "having trust" is so easily stated as the key to keeping a relationship healthy, and used as a blanket statement as the key to basically any other level of human relationship.

I feel like you don't understand trust. There's a reason people who have some swear by it but people who don't question whether it exists.

People distrust because they've been hurt and they're afraid of getting hurt again. The fundamental problem is that a relationship is nothing more than a protracted opportunity to get hurt. This is why courtship is so laden with subtext and hidden meaning: both parties are attempting to gauge each other's trustworthiness.

Trust is the most unstable thing I can think of! I'd be better off building my relationships on foundations of stale popcorn.

Here's the shitty part, from your perspective: if you aren't willing to lay your heart open to be gutted by someone else, you will find yourself engaging other people similarly distrustful. That's fine if you're just ridesharing but if you want to build a life together, your "foundation" is every bit as "stale popcorn" as you suggest.

"What of trust?" If you are with someone who shares your dreams, forgives your trespasses and protects your weaknesses, you are magnified. If you are with someone who doesn't, you're just sharing space.

And if all you've ever done is share space, it's no wonder you don't understand trust.