Manbearcat
Legend
If I get what you're going for, you're saying that people can't readily tell correlation from causation, and that they do all kinds of things regardless of whether those things are truly useful or not. Which has some truth to it. However, on aggregate, people can make some useful observations (for instance, it was accurately determined that white willow bark relieved pain long before we knew that salicylates inhibited cyclooxygenases). To be fair, they can also be wrong.
To go back to a point above, if people were truly as clueless as you're implying, they would probably be quaffing worthless nonmagical potions on a regular basis believing they were just as good as the real thing. But that doesn't happen much in my games. Does it happen in yours?
If not, there has to be some rationale for how the characters are so effectual in their actions.
Though I'm not taking a position of general or insidious cluelessness amongst a populace, You have the thrust of my post right. I would maybe say:
1) The physical body's actual reaction to it's perceived betterment (placebo) is extraordinary (and revealing itself to be even more profound as the science matures).
2). Common sense is (that is 1st order coupling and derivative having primacy in the role of phenomena we interact with and subsequently attempt to predict based on that shallow analysis) mostly a myth. The complex organisms and phenomenon we interact with and perceive are governed by all manner of 2nd and 3rd order functions that defy the silver bullet of common sense we so deeply hope persists (due to our inherent fragility and attendant insecurities) so we can make decisions that we are comfortable with while, inevitably, having very imperfect information.
Beyond that thesis, for my home game, genre expectations bears these things out. As GM, I'm mostly concerned with on-screen thematic conflict (the right NOW) and relentlessly challenging my players with the material and adversity they're interested in engaging with (no forays into thematically benign setting material to prove a "living, breathing world"). We resolve conflicts, things escalate and play (and story) naturally emerges due to that inertia. Ultimately, we find out what happens. Snake Oil Salesman selling worthless potions isn't a trope that we engage with. My campaigns bear no resemblance to the tropes of Gygaxian play. If players secure a stray potion, finding out whether the potion is cursed or worthless will never have a moment of spotlight. It works. Vendor investigation, haggling, or interrogation are not tropes we enjoy. We move on to the conflicts we enjoy. If they run across a curse or something such as that, it will be centrally relevant to a conflict that one of the players have signaled they wish to engage with.
There are a lot of genre and play style agenda assumptions (eg; pacing, player authority and stance fluctuation, tactical or strategic primacy, GMing principles, system complexity) that are just mismatches for our various tables. For example, low fantasy focused on setting exploration is a very different aesthetic (theoretically and in play) than high fantasy focused on addressing theme through serial, conflict-charged scenes. A hybrid of the two is different from both.