Here's why:

* Electron Transport Chain: The electron transport chain, the final stage of cellular respiration, occurs within the inner mitochondrial membrane. This process uses the energy stored in the electron carriers NADH and FADH2 to pump protons across the membrane, creating a proton gradient. This gradient then drives ATP production through ATP synthase.

* Oxidative Phosphorylation: This process, which occurs in the mitochondria, is the main mechanism by which ATP is generated from the energy released during the breakdown of glucose. It is fueled by the electron transport chain.

While some ATP is produced during glycolysis (in the cytoplasm) and the Krebs cycle (in the mitochondrial matrix), the majority of ATP is generated through oxidative phosphorylation within the mitochondria.