Because THEY WILL KILL YOU! Or they'll just shatter into a million harmless pieces like safety glass is supposed to and everyone walks away a bit stunned but unharmed.
I'm not entirely clear on what's happening here. It looks like someone is trying to come in the wrong way. I like how the person on the other side of the door just walks away like this never happened. Stay classy, door killer.
Check out Freak Accidents for more -- you guessed it -- freak accidents. You ghoul.