Knock, Knock...Who's There?

I have been spending an inordinate amount of time in doctors' offices this summer as a result of enough physical malfunctions to make the drug reps salivate. There's been a knee operation, an upcoming shock treatment ("Gimme, Gimme Shock Treatment") fort a heart issue, therapy appointments, and today: a repeat visit to the orthopedist because my knee surgery incision split open during therapy. My reaction to seeing blood come out of my surgically-repaired knee while taking a blood thinner was, well, worthy of its own blog entry. Stay tuned for the riveting story. For now, I wish to deal with a question that popped into my feverish brain today while being repaired a second time from the scalpel:

Why do doctors and nurses always knock before entering an examination room?

I get that when I go to visit someone's home I should announce my arrival with a doorbell chime or a knuckle-knocking, but I see the doctor's office as different for several reasons:
1. My chart is on the door. He or she KNOWS I'm in there. He or she KNOWS I'm waiting, not vacuuming, going to the bathroom, or talking on the phone to my broker or anything else that would make his or her coming in an intrusion.
2. What would I possibly be DOING in the room that they needed to warn me of their impending presence? Playing with the various anatomical models scattered around? Changing the screen saver on their computer screen? Pocketing anything I might be willing to pawn? Picking my nose or other orifice? Scratching somewhere that might be embarrassing to the incoming doctor or nurse? Unlikely.
3. Seriously. If I'm sitting there in one of those gown with no back, would I be cavorting around the room risking the full monty at any moment? NO.
4. Is there a magazine in there I shouldn't be reading? Something containing news from, say, this YEAR?
5. Do they want to make sure I'm not dozing so as to maximize the face time?

These are quasi-serious questions on my part though I'm sure the answer would be something as simple as "just being polite."
I would have asked today but I was so upset to have a newly-lacerated knee that I spent most of the time giving the practitioner incredulous looks that implied, "I don't believe a word of what you're saying. No stitches...just steri-strips and an ace bandage? Hell, I could have done that myself."
I hope I didn't hurt his professional feelings, but I was beginning to sweat and breathe shallowly again, so I had to get out of there with my question unanswered.