It may be that a rock is very complex, but it is random complexity. It has no context - it is just noise - so does not indicate design. In the same way, a random page of text, while complex in many ways, would be less indicative of design than a short sentence in an understandable language.
What about a galaxy or a hurricane? Do their beautifully complex paterns of spiralling arms imply a designer? (The complexity in this case isn`t random)