Some people say its the malpractice liability insurance. Some say its because one third of the money gets burned up by admin costs. Some blame pharmaceutical companies for pressuring consumers to demand prescription meds for marginal irritations ("Do you worry that you blink too often? Or not often enough? Are you thinking about blinking right now? Ask your doctor about prescription Xyzzyor.")
My theory is that a lot of it is down to bored doctors messing with patients' minds. Like this conversation in the doctor's lounge:
Bob: Hey, Ted. I just sent that guy for an MRI on his head.
Ted: Really? What does he have?
Bob: Ear infection. But he looked anxious so I decided to screw with him.
Ted: Hah! And did you tell him "It's probably nothing" and then do that face?
Bob: Yep. And then I said "There's a small chance that the scan itself can induce brain cancer. But of course, it's your decision."
At least, that's what I'd probably do if I were a doctor...