The point is, amazons were designed to be cheap to build, somewhat maneuverable, and able to carry an avionics package plus a little painting laser to simulate a weapon, as well as to blow up rather quickly under fire.
In order to make them combat effective, you'd have to add armor (increasing weight, necessitating bigger engines), shields (same), enough room to store a worthwhile weapons load (same, again), a more advanced avionics pack that can deal with real battlefield conditions without shorting out. In the end, you'd have a design that was an Amazon in name only; Considering the large amounts of old bomber spaceframes the GTVA has laying around in mothballs, converting them to drones by fitting a more advanced autopilot is probably cheaper.
That does not mean that there aren't going to be purpose-built drones at some point, but for now, the GTVA has decided to use up the reserves they do have instead.