What other logical reason could there be? They must enjoy getting caught with their pants down literally or something because when I'm changing in a fitting room and I hear the knock on the door and the "Is anybody in here?" I instantly panic and blurt out some noise to stop them from coming in. On a few rare occasions I'm able to form them into words like "STOP" or "I'm in here!" but usually I'm just so worried about being walked in on that I figure any noise is better than nothing. And, to this day I have yet to be walked in on at a public dressing room.
I've walked in on plenty of people though, and it's uncomfortable, and would have never happen if people would just answer the warnings.
I almost googled videos for dressing room pranks
but luckily my brain kicked in before I did.
That could have been bad....