Most biologists even those who pursue entomology spend very little time studying or training with spiders. Most people see two spiders like the ones mentioned and they just can't see the differences. They don't see slight variations in the shapes of the abdomen or the carapace. They are oblivious to major differences in coloring or banding present on the ventral views of two different species that may look almost identical from a dorsal view. There is differentiation between chelicerae or maybe even the numbers or spacing of the eyes. In some species of spider there is sexual dimorphism where a lay person could look at two spiders of the same species and think they are radically different species but are actually only different genders.

Someone who works primarily with vertebrates just doesn't have the knowledge to make a proper identification, unless its a knee jerk one.