I think they do this for the same reason that doctors and agencies do - because of the belief that we're hypochondriacs. The instinctive response of most people to hypochondria is to try to make the patient's life hell while they continue to claim that they're ill, to give them an incentive to change their behaviour. In my experience those feeling any sense of responsibility for the patient's suffering (i.e. doctors and family members) will be most likely to do this.
I'm afraid this is spot on in my experience too; there's a 'cruel to be kind' philosophy going on, that many people seem to have taken on board, it's just so tragic. Noticing this in some of my friends and family, and noticing that it's often some of the most caring people who treat us the worst, has helped me to realise that many of the worst authorities who cause us the most harm are also quite likely doing a good thing in their own messed-up way of thinking. It would be easier to understand if it were just really nasty people who were mistreating us, it's the tragedy that people do so from well-intentioned but hopelessly misguided motives that is really hard to deal with. And the circular reasoning and the completely unfalsifiable nature of psychosomatic theory makes the whole mentality seemingly impossible to break down.
I don't really know of any solution to it all. I can only think of two things that have brought some degree of progress for me.
One was: I spent a week with my dad where I undertook to follow his instructions precisely, in all matters, speaking to him by phone at the start and end of every day, telling him what had happened, and getting his instructions what to do. At the start of the week he was very confident. By day 4 he was at his wit's end, by the end of the week he had given up completely on solving the problem - but he understood how bad it was and that it wasn't the simple kind of problem he had assumed! That week cost me hundreds of pounds in equipment, clothing, bedding etc that I have never been able to use, and it took me some weeks to recover from what I went through. But it proved a worthwhile investment in the long run, to get a much greater degree of understanding from him, which has been really valuable for me.
Another thing that has occasionally made a difference has been when people have happened to see me at my worst, or when they have happened to see one or other of my more gross or undeniable symptoms in action, or when they have happened to observe an incident where I've been unable to do something that they know for sure I really, really wanted to do. Being confronted directly by some of our harsher realities can be a really scary eye-opener for people...but it's not something one can engineer I'm afraid.
Other than that, I would generally ask: How important is this person to you? There's not much point banging your head against a brick wall; often it makes more sense just to move on and find different people who
do understand. If the person
is that important to you, it might be worthwhile not even trying to confront the issue sometimes - trying too hard to change somebody's mind can end up destroying the relationship; maybe you can get by OK with them not understanding...though that's only possible in very superficial relationships. Finally, it's vitally important to try to stay calm. It can be the most frustrating thing in the world, but if you lose your cool, you generally lose the battle. Assertiveness comes in handy here: just describing, simply, what you experience; focusing on the most obviously physical symptoms, avoiding areas that get mired in a vicious circle kind of argument.
I
think that last paragraph is good advice...I only wish I were better at following it!...