What causes humans to be physically weak compared to animals like gorillas?
Evolutionary tools that are not needed for increased survival rate of offspring of a certain lineage deteriorate with time. The deterioration might have a form of gene expression changes or mutation accumulation. For example, gene MYH16 encodes a protein that facilitates strong jaw muscles in primates, but humans carry a mutated gene that had lost its function (Stedman et al. 2004. Nature). We cannot bite hard, but we can articulate.

With respect to muscle strength, the common ancestor of apes and humans was likely stronger than we are now. Their descendants that formed a lineage diverging towards humans increased their survival by being smarter rather than stronger than their contemporaries.