Realize here that we're not talking about software running on a server in a nice temperature controlled room. This is software controlling hardware that is under constant vibration, will get sticky, or bend, or break, or ice up under variable conditions - hot, dry, rainy, wind gusts as it goes from behind a building to crossing an intersection. There is a mind boggling number of things that can go wrong when you're controlling a device in the 'real world.'
Even if the software is perfect there is still a large number of variables to account for and most of them can't be controlled. There's a case for UAVs and I would love to get involved with them, but building a reliable UAV and properly maintaining it would almost certainly cost too much to have it deliver tacos. Unless you're willing to spend $250 to avoid walking a few blocks.
No one argues this is not true, it's the premise that's unlikely.
Can you offer more details?