In Sweden, as in most of the EU, electric bicycles are required by law to reduce the assistance as the speed increases and to cut out entirely above 25 km/h. So an electric bike wouldn't be much help to a courier, who would mostly be going faster than 25 km/h. (A faster vehicle would come under the moped regulations and the rider would need a license and insurance.)
Last but not least, esp in dense traffic: With a good bike you can accelerate much faster than you can economically on a scooter.