Some ICE vehicles don't have a transmission! The most important reason for ICE engines to have multiple gears is low speed operation and moving from a stopped position.
First - ICE engines have a minimum running speed, usually between 650-800 rpm. Driving in a gear that runs the engine slower than this can cause the engine to stop running or damage it. The vehicle needs to be able to operate at low speeds, which requires a low gear ratio. Operating at high speeds requires a higher gear ratio so the engine isn't damaged from running at extremely high rpm (efficiency is also an important factor, but secondary).
Second - Motor vehicles with ICE engines (ignoring hybrid assist features) need to be able to move away from a stop with the engine running, and without allowing the engine rpm to drop below idle. This requires a clutch mechanism to disconnnect the running engine from the rest of the drivetrain, and to slip as the vehicle's speed rises to a point where it's safe to lock the engine's rotation speed to the drivetrain's rotation speed. Manual transmissions come with manually operated clutches, newer automatics have automatically operated clutches, and old automatics have a hydraulical coupler that slips at low engine speeds.
Massive vehicles have an extremely low first gear, so they can get going from a stop. Passenger vehicles have a first gear low enough to operate in places like a parking lot. Top fuel drag cars only have one speed; a massive clutch controls engine power transfer to the wheels.
Electric motor vehicles don't have the low speed operation problems that ICE cars have, or anywhere near the level of inertial wear and cacaphony at high rpm, so the transmission can be optional. Transmissions are expensive, and a vehicle in the EV market in its current state doesn't need that level of optimization to be competitive.