Here's why:

* Fermentation is an anaerobic process. This means it occurs in the absence of oxygen.

* Fermentation uses glycolysis to break down glucose. This process produces a small amount of ATP (energy) and pyruvate.

* Pyruvate is then converted into various products depending on the type of fermentation. Examples include lactic acid (in lactic acid fermentation) or ethanol and carbon dioxide (in alcoholic fermentation).

In contrast, cellular respiration, which is an aerobic process, requires oxygen. Oxygen acts as the final electron acceptor in the electron transport chain, which generates the majority of ATP produced from glucose.

Therefore, fermentation does not use oxygen as it is a metabolic pathway that takes place without the presence of oxygen.