People end up leaving stuff in the boxes all the time. probably, they forget it is there. As such, the boxes tend to repeatedly fill up. Someone, I don't know who, empties stuff out from time to time.
You can avoid having your stuff removed due to this, by using boxes with labels that state the dates of use. Boxes are MUCH better, as they can stack and used a smaller footprint. You can't really do this labeling with a bag/sack.