Because drones are many years away from operating in such a complex environment. There are basically two ways you can think of drones operating: remotely piloted, where a pilot is just sitting somewhere other than in a cockpit; or autonomous, where the drone executes a programmed plan, either a very simple one that is scripted or one with increasing degrees of judgment exercised by the drone itself.
Right now, the state of the technology is that we can do really simple autonomy (drone, you are directed to fly this pattern, do this simple thing, and return home) and totally unable to do anything complex (drone, fly in this region, but if you see something suspicious, go and investigate it and make a call whether to use a weapon). Therefore, anything that is even a little complex right now falls into the remotely piloted bin.
We can reasonably expect that over the next 10-20 years, our technology and confidence in autonomy will increase so that a drone can fly a mission to drop a bomb on a predetermined target, or maybe even a moving target under strict parameters, but this is really, really hard stuff. What if the drone gets to the munitions factory it is supposed to bomb, and there’s a school bus of 5th graders taking a tour? Can computers make the judgment not to deploy the weapon?
Okay, you’re asking, so why don’t we just have pilots fly those missions? Why not have a remotely piloted drone replace the A-10? Because remotely piloted vehicles depend on radio signals, typically from satellites, to command them. It is trivial to jam these signals. Every significant military has enough electronic warfare capability to put out signals strong enough to create real problems in satellite communications near the battlefield. For example, the idea of Predator-like UAVs operating anywhere near, say, hostile Russian forces today or in the near future is just a joke. Those Predators would be mincemeat.
We will surely get there some day, but the ideas of either having a drone operate autonomously with weapons near friendly troops, or having a drone with weapons have assured communications near enemy troops, is a really, really difficult problem.