Here's why:

* Mitochondria and Energy: Mitochondria are the powerhouses of cells. They produce ATP (adenosine triphosphate), the primary energy currency of the cell.

* Contraction and Muscle Cells: Rapid contraction is primarily associated with muscle cells. Muscle cells require a lot of energy to contract quickly, and therefore have a high concentration of mitochondria.

* Other Animal Cells: While other animal cells need energy, they don't contract quickly. For example, nerve cells rely heavily on mitochondria, but their primary function is communication, not rapid movement.

In summary: While all animal cells need mitochondria for energy, it's not the presence of many mitochondria that directly leads to the ability to contract quickly. Muscle cells are specialized for rapid contraction and have a high concentration of mitochondria to support this function.