I enjoy surprising people with room service breakfasts at 5:30 am when those particular door hang cards are available. ;)

It's worth noting that hotels just assume that a certain # of DND signs will go missing at the end of the year (like pens, pads, umbrellas, etc).

I'd never take one off someone else's door but I have been known to take them in general.

why do these things even still exist?

much prefer the electronically operated ones , which are rare to find

I guess I'm a bit old-school WRT to DND signs, as I'd always prefer the physical ones to the electric ones (even at the best hotels, I've had the buttons short circuit/not work midway thru the night).

Renaissance Seattle has the electronic kind. It's basically a small red light you can turn on from the inside of your room that tells housekeeping to not disturb you. No DND signs needed.

Probably because most people don't understand what light the DND light is in the room so just turn it on automatically and then wonder why room service never comes.