Why is cellulose considered to be a complex carbohydrate?

Chitin and cellulose are both polysaccharides

Polysaccharide are the complex form of carbohydrates.

A starch molecule is formed when many disaccharide molecules are combined.  Starch is often produced in plants as a means of storing energy. The two other molecules formed when multiple disaccharide molecules are combined are cellulose and glycogen. The cell walls in plants are made up of cellulose. Glycogen is a carbohydrate energy source. 
A disaccharide molecule is formed when two monosaccharide molecules are joined together. Maltose, sucrose and lactose are the most important disaccharides. A monosaccharide molecule is a simple sugar, for example, glucose.