"Tesla doesn't make slow cars" - Elon, simple as that 😂
Jokes aside, the motor is probably made conservatively more capable than necessary to cover many different scenarios, think mountainous terrains, varying climates, etc.
But more importantly, I imagine they will also use the same motor design for Robovan (multi-motor config, of course) to streamline extreme-scale production, which incentivizes a slightly more capable motor than the absolute minimum needed for Cybercab alone.