Wouldnt it be nice if you could just levitate the stones up to where you need them. This one looks like it is on its way up to Joe, but it's caught in time on its way down. Most stones are on their way down. They are normally moving very slow in that direction, unless of course if you push them or throw them.
Stones like to be heavy. They like to be close to the ground. It is this quality that makes them useful. It is this consistencey that makes them stay where they are in a dry stone wall, in an arch or a cairn.
People are not always as reliable. They may change their minds on you. One day they may think that it is very cool to be given an opportunity to learn how to build challenging structures like the one above, and the next day decide that it's wrong to teach anyone else. Luckily stones are more consistent.
Stones, if fitted together properly, even though they are 'pulled' in that direction, will support each other and don't end up being in that great a rush to get to the bottom.