At a minimum, without even shipping more items, the result of gig couriers vs. fedex/ups is 2.7x the number of vehicles on the road (due to size difference).
It also gets around legal liability costs, since you can just create and destroy hundreds of gig courier corporations around the country when issues arise from "pushing the envelope" in terms of safety and policy.
Can you imagine the world we would live in if 90% of current retailers went to the Amazon ecommerce + gig courier model? Our infrastructure (primarily roads) would immediately fail.