Because an electric car with a single ratio will typically have a much higher power to weight ratio. Let's be generous and say the bike has 1kW and weighs 200 pounds with a rider, that's 5W per pound. A Tesla Model S has 568kW and weighs about 5000 pounds, that's 114W per pound. With so much extra power available it can be geared much higher for a decent top speed whilst still being able to accelerate quickly. Also the motors will tend to spin much quicker than those used in bikes.
Regarding the Teslas I do believe some models actually use multiple motors with different ratios, one more suited to accelerating and one for high speed cruising.