You may notice in the first photo that all the rocks have been placed onto aragocrete bases. That gives them a flat, stable bottom to sit on. It distributes the weight and since it is completely flat it doesn't allow detritus to get under the rock. Plus, it gives a more realistic appearance, especially on BB. I have never seen rocks rising up from points when diving! The ability to assemble the rocks dry, create a flat bottom and attach them together with concrete is a BIG advantage that you just can't get with live rock. Of course, there is some live rock on top- about 1/3 of the total.
Allen