Because the world is full of half-wits that have no care or even awareness of others.
A few times contractors have left empty water bottles and fast food wrappers at my place...to me it is an indication of sloppy management when workers think it is ok to do stuff like that. I even had some concrete workers write gang signs in wet concrete because they knew I was soon over-laying the concrete with flagstone....but still that's pretty lame if you ask me.
