Drones have to be specifically designed and manufactured to pass the FAA's certification program. We've all seen capable drones from Amazon's marketing, but does it please the FAA? A drone can't do the job until the FAA signs off on that make/model. It's not easy to do and requires developing a craft with all sorts of safety features.
Companies are making progress, and I think the most insightful videos to watch on the topic come from Zipline. They're already operating in Rwanda with support from their government. They have a streamlined service and make 100s of deliveries per day.