Every drug mule is a potential informant, and one more possible leak of the to/from information about the trade.

With an autonomous step in the chain, there is no human who knows where the craft is going to drive to. If anyone picks it out of the water it can be programmed to erase information about where it's headed towards.

The cartels likely already see these as "automated", given that, like you said, the mules are very low value to them. Maybe not automated, but definitely 'autonomous'.