Why are lipids not considered polymers?
Polymers are chains of smaller molecules that can interlink, while lipids have a starter molecule, like a fatty acid, and build long chains through a chemical reaction, like dehydration.
Polymers are chains of subunit molecules that are all similar in a way that allows the subunits to continue building longer macromolecules on either end.
By contrast, lipids are chains of carbohydrates that are chemically bonded (such as by dehydration) to each other into a new, larger molecule, and have a unique starting molecule that is different from the subunits. So, lipids only grow in one direction.
Also, there is no natural fixed size to a polymer, so a process terminates it in living organisms. Some lipids have a natural limit, as additional hydrocarbons might not naturally dehydrate at the open end. For example, such a lipid is cholesterol.
Dehydration is the process of creating a bond between two molecules to create a new, larger molecule by taking one hydrogen from one molecule, and an OH from the other, creating one molecule of free water.