Why NaCl does not exist as molecules but aggregates of oppositely charged ions ?

In case of sodium crystal each Na⁺ ion is surrounded by six Cl^– ions and vice versa. It is difficult to identify that which Na⁺ ion belong to a given Cl^– ion or which Cl^– ion belong to a given Na⁺ ion . So, we select a combination of Na⁺ and Cl^– as a formula unit. The formula unit of an ionic compound is the simple collection of cations and anions which is electrically neutral. Formula unit does not exist as a separate entity. So due to this ionic compound like NaCl do not exist as discrete molecule in the crystal lattice but exist as aggregates of a very large number of oppositely charged ions.