They are faster than cars
Amazon did a test to see which method of delivery is fastest, by holding time trials in New York City. The bicycle couriers were given a package and an address from their bosses, and had to deliver the package within a certain time limit and make it back to the office.
All around the world, every major city that has started using cyclists for deliveries has seen for themselves that cyclists are just as fast as, or faster than using cars. This is because of a number reasons, but primarily it’s because of traffic. Cyclists are less susceptible to traffic, and can generally move more, and travel at faster speeds during rush hour.