Why would they get an expensive self driving car when they can buy a much cheaper drone that's much easier to program a flight path for and doesn't come with pesky pedestrian avoidance algorithms? A shrapnel bomb above a crowd could be much better at killing people because it's got a line of site path to much more of the crowd.

They could do this for the last several years but rarely (or never) do, so I'm not anticipating an upsurge.