All it takes is one person to really mess it up. It's kind of like breaking the ice -- the barrier to actually making the area go from "clean" to "not clean" is much higher than just making it go from "messy" to "slightly messier".

In an office with a dozen or more people there's a pretty high chance there's at least one person who is lazy, "too important" and/or "too busy" to clean up after themselves.