Why is ammonium hydroxide no longer considered a separate substance in ammonia solution?

Although the formula ${\text{NH}}_4\text{OH}$ is often used to represent aqueous ammonia, it is not considered a separate molecular species because there is no appreciable amount of ${\text{NH}}_4\text{OH}$ present in the solution.1

Instead, an ammonia solution consists primarily of water (${\text{H}}_2\text{O}$) and ammonia (${\text{NH}}_3$) molecules linked by intermolecular hydrogen bonds. While ammonia does react with water to form a basic solution, the equilibrium—represented as ${\text{NH}}_3+{\text{H}}_2\text{O}\rightleftharpoons {\text{NH}}_4^{+}+{\text{OH}}^{-}$—favors the reactants. Consequently, the solution contains only small quantities of ammonium ions (${\text{NH}}_4^{+}$) and hydroxide ions (${\text{OH}}^{-}$) rather than a distinct ${\text{NH}}_4\text{OH}$ molecule.12