I think it's not so much the weight of the pilot but how requiring a pilot effects the upper bound of 'delivers per day'.

- Pilot can't reasonably fly more than 8-10hrs a day Need at least 3 pilots per aircraft for 24/7 delivery support.

- Pilot needs a salary, drastically increasing the OpEx of each drone (vs. "just maintenance and electricity")

- Ability to train & hire pilots limits total fleet size.

- Pilots require insurance, which is likely a nontrivial amount. (depending on regulatory environment, of course)

Imagine being able to build 1,000 drones in a factory. Can you as easily hire, train & pay 3,000 pilots? Probably not.