As one woman explained it to me recently in front of me when she asked if I could get her a bin -

These things are filthy and are never washed, I am not touching them.

I think that's one reason so many people won't stack them once they get their stuff, they want as little contact as possible.

Plus, it's hard to figure out what to do with them. Some airports it's pretty obvious, there's a cart for them at the end of the rollers. But at others they get stacked on top of the guard over the belt, they go on the floor, etc. It's just easier to run for the plane then it is to figure it out for some.