Why are the labels frog and toad not strict scientific categories?

The labels "frog" and "toad" are not strict scientific categories because there is no taxonomic basis for distinguishing between them; both groups are tailless amphibians that belong to the same order, Anura.12

In a biological sense, all toads are considered frogs. The terms are instead common descriptors based on superficial physical characteristics and habits. Generally, "frog" is applied to species with smooth, moist skin and long hind legs adapted for leaping, while "toad" is used for robust, short-legged forms with dry, warty-looking skin that are often found further from water.123

However, these descriptors are highly inconsistent. The application of the label "toad" is so uneven that closely related species within the same family may be labeled differently, and some species are even referred to by both names. Only members of the family Bufonidae are formally recognized as "true toads," yet many other unrelated anurans are colloquially called toads despite lacking a formal taxonomic classification that separates them from frogs.134