Which they tell you about before you ever step on the trail, and tell you to take everything you bring in back out. It's not hard in the slightest to put your garbage in your pack/pockets till you leave, but people are selfish, lazy fucks who can't even do the minimum and respect the space.
The ones they have are full.
Which is why they don't have them in a lot of places. Because being there and full is worse than not having them at all, since people just keep throwing shit on the pile.