Because with cellular respiration (aerobic), the nutrients are completely oxidized and broken down to carbon dioxide and water. Fermentation is not accompanied by the complete oxidation of glucose and its breakdown into inorganic substances; therefore, a significant part of the energy remains contained in the chemical bonds of the final fermentation products (lactic acid, ethyl alcohol, etc.)
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.