You won't jump into a modern electric car and row through a number of gears as you race down the road, which begs the question: why is it that electric cars usually have only one gear?
It's but another topic for Jason Fenske of Engineering Explained to cover for us, so throw on your learning caps. Foremost, the broad answer to the question is that electric motors don't require a number of gears due to their high-revving nature. Electric motors are also efficient across the rpm range and produce mounds of torque from a low rpm.
An internal-combustion engine requires multiple gears to reach a top speed, but engineers can gear an electric motor specifically for a desired top speed. Jason uses Formula E as a proper example. A standard Formula E racer's electric motor can rev to 20,000 rpm. By doing some math to solve for the racer's gear ratio, it works out to show a single gear with an 11.4:1 ratio can reach 225 kph (roughly 140 mph) while spinning to 20,000 rpm. Therefore, there's no need to add cogs if the single gear can accomplish the Formula E racer's top speed.
Take into account most production electric cars are governed at around 100 mph, it's easy to see why a single gear is more than enough.
It's worth noting exceptions do exist. The original Tesla Roadster began life with a 2-speed transmission, but the company eventually did replace it with a single-speed unit.