Here's why:

* Cellular Respiration: Cells normally use oxygen to break down glucose (sugar) for energy. This process, called aerobic respiration, is highly efficient, producing a lot of ATP (the cell's energy currency).

* Anaerobic Conditions: When oxygen is scarce, cells can't rely on aerobic respiration. Instead, they switch to fermentation, a less efficient process that produces much less ATP.

* Fermentation Products: Fermentation uses different pathways to break down glucose and regenerate the electron carrier molecules (NAD+) needed for glycolysis (the first stage of cellular respiration). This process produces byproducts like lactic acid (in humans) or ethanol and carbon dioxide (in yeast).

So, while fermentation is a way for cells to survive without oxygen, it's not the most efficient way to generate energy.