Ah, so you've never had a job as a subject matter expert speaking to a member of the general public i.e. GP, Pharmacist, Mechanic, IT Technician, etc.?
People have no idea about the complexity and scale of the world. Just how complicated the logistics chain for something like Kentucky Fried Chicken is... "hOw DoEs A cHiCkEn ShoP rUn OuT oF ChIcKeN!?!"
The best is when you are a consultant, as people will actually bring you in at great expense or fly in from around the world to disregard your opinion.
Now I just go on Lemmy to get my opinion about those same things (and others) disregarded from the comfort of my toilet.
Emotional validation of a sense of the world being against them. A looot of people are significantly happier to be proven right about the world being horrible to them personally than change their own outlook or behavior. It turns out our evolutionary psychology STRONGLY favors not having to reprogram anything and will generally move to protect your existing psychological coping scheme before trying behavioral changes. It makes 0 sense but I watch it happen all. the. time.
The only real way to force someone out of it (and even that only works like half the time) is inpatient care where you can control the environment enough to be able to be nice to them safely. In the community often the only safe way to deal with people this delusional is exactly what this doctors' office did: refuse service / tresspass or if you're an individual just give them a clear berth. And even inpatient care is only worth it if there's an actual psychiatric disorder involved. If they're just a prick its going to cause more problems than it solves.
Bet they spoke in that calm psycho Karen voice and made veiled threats. These people think that if you don’t shout and don’t make direct threats that you can say anything to a person.
It's always intimidating talking to a clearly unhinged person. If they're a stranger, you don't know whether they will resort to violence if provoked. You only know they don't observe regular social conventions.