When you're hooked on one door code in particular

from here (image source)

Not only is it easy to tell what numbers are in the code, the order seems like it should be easy to guess as well. I feel confident that it's 1709 - it can be entered in a relatively fluid motion that saves time and effort. You might think that the door code should be changed on a more regular basis, but that would invite people to write it down and that that slip of paper could get lost or stolen and compromise the security. Instead the keypad should be better maintained and replaced when wear patterns start to give away the code.