Here's why:

* Nucleotides are the building blocks of nucleic acids (DNA and RNA). They are composed of a sugar, a phosphate group, and a nitrogenous base.

* Proteins are made up of amino acids. They are completely different molecules with different structures and functions.

* Nucleotides are synthesized by enzymes. These enzymes are proteins, but they are not used as building blocks for nucleotides. Instead, enzymes facilitate the assembly of nucleotides from smaller components.

Think of it this way: You can't build a house with bricks if you only have wood. Proteins and nucleotides are different building materials, used to create different structures.