There is this confusion over the word “drone”.

What you say is absolutely true about quadrocopters and octocopters. It is much less true about fixed wing drones.

You don’t have to go all the way to a Global Hawk to get reasonable carrying capacity. Even hobby sized stuff could be used for this reasonably well. Parts are easy to acquire, they are easy to conceal, you can launch them with a catapult like setup, they can drop off their load and fly back autonomously.

I bet that it is already happening or at least there are smugglers experimenting with the tech.