> Why is it so hard to believe that the mind controls the body at a cellular level?

It does, obviously. Contracting a voluntary muscle requires work that is done at the cellular level. A signal travels from the brain and ends up translated into a chemical message at the cellular level which contracts muscle fibers.

The brain is even able to address a specific set of cells that correspond to a bundle of specific nerve fibers for that muscle.

Nervous-system signaling involves the cellular level: changing electric potentials, ions flowing across cell membranes, and and such.