To add to "capital-intensive": the regulatory framework right now (in the US) for drone delivery is uncertain at best. While most of the rules & regs to enable

To add to "capital-intensive": the regulatory framework right now (in the US) for drone delivery is uncertain at best. While most of the rules & regs to enable drone delivery is already finalized & published, a few very important ones (such as BVLOS - beyond visual line-of-sight) have yet been finalized, and thus need one-off waivers from the FAA to allow. And getting those one-off waivers is what eats a lot of time & money.