RPGMemes

Finally, there's the hardcore simulationist angle, which I ignored completely above.

Many fantasy writers, whether that be books, shows, computer games, or RPG media... know what a human is and can do, and design the humans in their setting to comport with their experiences of what a human is from the real world.

It serves as a kind of grounding baseline, a foundation of familiarity that doesn't need any work from the audience. "Ah yes, a human. I know what that does." A useful starting point that ensures the audience has something low-concept and relatable in an otherwise high-concept offering.

With a simulationist mindset, humans are often the "default", and least "fantastical" of the species in the world, because there's an established preconception of what humans are like.

Other species can have supernatural dexterity and grace, or magic gifts, or strange relationships with the laws of physics, because they're made up, and the mundanity of the human serves as a mirror that highlights and emphasizes the fantastical elements of the other species.

This approach isn't so much an attempt to incentivise diversity as it is just our natural inclination to not interrogate the known and familiar. Of course humans don't live for a thousand years, or have supernatural strength, or an immunity to poisons, we know what humans are like.

It's a fairly small and easy leap to invent a new species and say "this species is immune to fire." It's a leap to ask "what if humans were immune to fire?", and many people never even consider playing with that, because in our minds, humans aren't fantastical creatures.