The technical argument is that these are potentially weapons of mass destruction, and the international community has thus far banned all other weapons of mass destruction.
What makes these different from previously banned weaponry is their potential to discriminate. You could say, “Only kill children,” and then add facial recognition software to the system.
Moreover, if these weapons are produced, they would unbalance the world’s geopolitics. Autonomous robotic weapons would be cheap and easy to produce. Some can be made with a 3-D printer, and they could easily fall into the hands of terrorists.
Another thing that makes them terribly destabilizing is that with such weapons, it would be difficult to know the source of an attack. This has already happened in the current conflict in Syria. Just last year, there was a drone attack on a Russian-Syrian base, and we don’t know who was actually behind it.