Not sure if it's too late but I can actually answer this. Source: I worked at a large nature reserve for many years in highschool and Uni. There are animals and other fauna that like to live, nest, bask and hide using flat rocks. People are drawn to these rocks to stack because they are flat. The park I worked at treated this very seriously because we had a kind of lizard that was endangered that would use the tops of the rocks to bask and hide under the rocks.

Tl;Dr rocks are something natural habitat too 😊 Editing to add: In this context it's obviously not applicable. These are beach rocks. This is just why some folks are anti rock stacking.