Here's why:

* Metallic bonding: Copper atoms have a relatively weak hold on their outer electrons. These electrons are delocalized and form a "sea" of electrons that are shared throughout the entire structure.

* Strong attraction: The positively charged copper ions are attracted to the negatively charged electron sea, creating a strong cohesive force.

* Lattice structure: This strong attraction results in a regular, repeating arrangement of copper atoms, forming a crystalline lattice structure.

Molecules are formed when atoms bond together through covalent bonds, sharing electrons. Copper does not form covalent bonds with itself, and therefore does not exist as molecules.