Why do we use the term "formula unit" rather than "molecule" when we refer to the simplest repeating unit of an ionic solid?

In order to understand the difference between Molecule and Formula Unit lets take one one example for each.

NH₃:

Ammonia is a covalent compound which contains three covalent bonds between N and three H's. One molecule of Ammonia can exist independently, so it is possible to find a single molecule of NH₃ comprising of one nitrogen and three hydrogen atoms bonded covalently.

NaCl:

Sodium chloride is made up of Na⁺ and Cl⁻ ions. Now, the formula NaCl is not molecular formula, instead it is a formula unit. Formula unit means that this compound consists of giant structure of Na⁺ ions and Cl⁻. There numbers can not be counted but the ratio of these two ions to each other is countable and for each Na⁻ ion there is one Cl⁻ ion. Hence, Formula unit is an Empirical formula of any Ionic compound.