Here's why:

* Muscle cells require a lot of energy: Muscles are responsible for movement, which requires a great deal of energy. Mitochondria are the powerhouses of the cell, producing ATP (adenosine triphosphate), the primary energy currency of the cell.

* Fat cells store energy: Fat cells primarily store energy in the form of triglycerides. While they do have mitochondria, they don't need as many as muscle cells because they aren't actively using energy for movement.

Therefore, muscle cells need many mitochondria to produce enough energy to support their high energy demands.