Because Rhino is an engineering tool above all.
As such, for CGI I consider it an overkill.
In Rhino to create such a rock you need to first draw the profile polylines, then extrude them, then trim them, then join them. One mistake and wrong accuracy set and you may spent hours to split and join to make it a closed solid.
In Blender you start with a box subdivide it multiple times and then all you need to do is drag the control points to match all profiles(pictures). You can even make it rougher as in Blender you can pull edges (functionality that may be possible in Rhino7 ). You can make the rock hairy if you extrude points. You can’t do this in Rhino (to my knowledge).
Oh yes, and there are different modifiers in Blender that will make your job much easier. And all this will take you (for this rock) from 15 min to 1 h with painting the texture and rendering