because they are children. i have worked with these sorts as well, leave some shit laying around leave a work area messy...you say something and they get all surprised and tell you 'whats the big deal?'. they have to be told to clean up after themselves, like a child. then they complain, like a child, now you are the dick because you didnt want to work around amidst the mess they created.
part of it too is that they know if they leave it long enough, someone that actual does care will probably clean it up for them. idk, maybe they have no control/power at home or in other aspects of life, and this is one thing they feel they can do as they please.