/sci/ - Science & Math

They're not. It's just that most "species" were defined based on their appearance rather than genetic relations, centuries ago before biologists and naturalists knew anything about genetics.

Take a look at the huge variety of foxes, for example. Excepting the urocyonids, all of these are extremely genetically similar and likely could interbreed if not for incompatibilities in habitat, behavior, and breeding season. By some biological definitions, they could be called variations on the same "species".

Compared to that, the various human races differ very little, more akin to regional/color variations between red foxes across a country, than between red foxes and fennec foxes from different continents.