Here's why:

* Plants are autotrophs: This means they can make their own food, unlike animals who are heterotrophs and must eat other organisms.

* Photosynthesis converts light energy into chemical energy: This chemical energy is stored in the form of glucose (a type of sugar), which is the plant's food source.

* Glucose provides energy for growth and other processes: Plants use the glucose they produce to grow, repair themselves, reproduce, and carry out all the other functions necessary for survival.

While animals eat plants, they are not directly consuming the product of photosynthesis. They are consuming the plant's tissues and stored energy, which were built up from the glucose produced through photosynthesis.