Because it is a simple sugar. It's called a monosacharide. A polysacharide (polymer, but better for this situation) would be something like starch. The difference between the two is that glucose is a single group of elements, or a molecule (C6H12O6 is the chemical compound) and a polymer would contain tons and tons of these sugar molecules.
Glucose is not a polymer because it is a single monomer and can combine with other glucose molecules to form polymers, such as starch and glycogen. Polymers consist of multiple monomer units, while glucose exists as a standalone molecule. Therefore, glucose qualifies only as a monosaccharide and not as a polymer.
;