One of the reasons is that a gearbox capable of withstanding the torque from an electric motor would need to be rather heavy. I suspect that if there are any efficiency gains to be had, they’d be offset by the increased mass and transmission losses. Not to mention the increased parts cost, manufacturing complexity and packaging challenges.
That is true, but I've also read that F1 (electric series?) was using transmissions at first and some stuck with them, while some didnt. Although, F1 has unlimited budget. Im sure using lower power motors would allow the usage of transmission and maintain enough power to be a daily commuter.
