That is completely dependent on the design of the engine. There is no reason that an electric motor cannot be hooked to a transmission. It just doesn't need it as badly because they can handle sustained high RPMs, and in many cases, are specifically designed for high RPM operations. So it makes sense to keep things simple, and leave out the transmission, and that is what we are seeing right now.

But as electric cars become more common, they will need to be able to handle all types of driving, including steep slopes, which is a very common reason to stay in a lower gear. There will be demand for such a function, even if not in every vehicle.