Electric vehicles do not add gearboxes mainly because they do not need them. The electric motor used in an electric vehicle has a much wider range of torque, and it can deliver maximum torque from zero rpm, unlike an internal combustion engine that needs to reach high rpms to deliver maximum torque. So, the torque of the electric motor can be used to its full potential in a single-gear transmission.
Another important point is that adding a gearbox would add complexity and cost to the electric vehicle, and it would also add additional weight, which would affect the vehicle’s performance and range. Additionally, a gearbox requires additional maintenance, making the vehicle less reliable and more expensive to own.
Furthermore, electric vehicles are designed to be more energy-efficient at high speeds than gasoline or diesel vehicles. In an electric vehicle, the electric motor is much more efficient than an internal combustion engine at converting energy (from the battery) into motion, and this efficiency is maintained even at high speeds. Therefore, adding a gearbox to an electric vehicle is not necessary to solve high-speed energy consumption problems.