I'd say the vast majority of people don't litter or trash things, but the few that do ruin things for everyone. It only takes a few bad apples (on in this case trashy people) to ruin something for everyone else.

Its like living with 4 roommates, but one of them leaves their shit everywhere. It could make the whole place look unkept for everyone else.

And because we generally don't want to pick up after other peoples shit/mess things may be left trashy for a long time before anything gets cleaned.