Here's why:

* High Energy Density: Carbohydrates, particularly in the form of starch in plants and glycogen in animals, can store a large amount of energy per unit of weight.

* Easy Breakdown: Carbohydrates can be easily broken down into glucose, a simple sugar that can be readily used by cells for energy production through cellular respiration.

* Efficient Transport: Glucose can be easily transported throughout the body, ensuring that energy is readily available to all cells.

While other biomolecules like fats and proteins can also be used for energy storage, carbohydrates are generally the primary source due to their advantages in terms of efficiency and accessibility.