Here's why:

* Structure: Glycogen is a highly branched polymer of glucose. Its structure is similar to amylopectin, a component of starch. The many branches and compact structure of glycogen create a large, complex molecule.

* Hydrophobicity: While glucose itself is soluble in water, the many glucose units in glycogen are linked by glycosidic bonds, which are essentially non-polar. The overall molecule is therefore relatively hydrophobic.

* Size: The large size of glycogen also contributes to its insolubility. The large molecule is too bulky to be effectively surrounded by water molecules, preventing proper hydration and dissolution.

Instead of dissolving in water, glycogen forms a suspension. This means that it forms small particles that are dispersed throughout the water, rather than dissolving completely. This is how glycogen is stored in the liver and muscles.

Note: Some sources may mention that glycogen is "partially soluble" in water, but this is referring to its ability to form a suspension, not true solubility.