Security and stupidity
Okay, I’ll rise to the [http://www.useit.com/alertbox/passwords.html bait].
You can’t be serious. There are often people seeing what you type, besides which, if this was adopted then people would _start_ looking. When I’m assisting someone on a computer and they are at a password prompt, I even go to the lengths of looking away – very pointedly moving my whole head not just my eyes so that they know I’m not looking.
Of course, I’m a sysadmin and a geek – if I _really_ wanted their password I probably could.