Just like aircraft they will be constantly grounded due to bad weather, helicopter are grounded regularly due to high winds, so if your a delivery service that would be pretty inefficient. Also bird strike, delivery trucks dont get stopped by that, and many people will be tempted to down a drone, so to save money on paying people to deliver goods, maybe they should factor in the possible loss of items and thus the loss of business, compared to just staying with what works, rather than trying to avoid paying people.