I mean with rigid plastic containers robots are 'pretty consistent' at it now.

The problem with things like cardboard boxes, especially at any size is internal weight distribution and deformation of the box. If you take someone that is pretty new to stacking boxes at a wearhouse and give them sloppy boxes (ones that bend or otherwise shift) they are going to be pretty slow at it for the first hour or so, then we'll internalize the play in the materials and start speeding up considerably while getting a nice result.

It's pretty amazing how evolution has optimized us for feedback sensing like this.