Why is direct nucleophilic substitution of an alkyl halide with NH₃ not a very useful method for preparing primary amines?

Question

Quizplus · Accepted Answer

Direct nucleophilic substitution of an alkyl halide with NH3 often leads to polyalkylation, where the initially formed primary amine can further react with more alkyl halide, leading to secondary, tertiary amines, and quaternary ammonium salts, resulting in a mixture of products.

Why Is Direct Nucleophilic Substitution of an Alkyl Halide