My understanding is that gearboxes trade torque for rotational speed, and electric motors have plenty of the later but with less torque than exploding hydrocarbons to push pistons. So my guess is that in addition to be unnecessary (electric motors can do a lot of rpm with constant torque), it might be really hard to make a manual transmission electric car drive well with more than a couple gears. And switching between them wouldn’t be anything like the experience of an ICE car.