Possibly because in the past there was a notion that doctors would lie to their patients "for their own good", and that the doctor's decision about your care was something you had to just accept.

The notion that patients could advocate for their own care arose during the boomers' lifetime.

It's not that strange an idea given the extent to which the pharmaceutical industry essentially bribes doctors to prescribe their drugs, or the frequency with which health care organizations defraud Medicare and insurance companies, fraud which often takes the form of unnecessary medical procedures. Blindly trusting any professional is the strange idea (I agree that constant distrust is probably unhelpful).

That's because a lot of doctors are untrustworthy (being people), and as the years roll by many have watched our loved ones suffer and die in horrifying pain

IT WILL HAPPEN TO YOU