Here's why:

* Glycogen is the storage form of glucose in animals. It's a branched polymer of glucose that's stored primarily in the liver and muscles.

* Plants store glucose as starch, a different type of polysaccharide.

Some bacteria and fungi can also synthesize glycogen, but this is less common.

So, while many organisms use glucose as a primary energy source, only animals (and some microbes) store it in the form of glycogen.