It is not sufficient to just be able to carry missiles in the air. Missiles only work in conjunction with the right sensor and radar systems to designate and guide it to the target.
So yes, you do need both the plane (with onboard sensor/radar) and missiles in order to hope for a kill.
Sensors and radars take more space, power and weight. So the drone will need to be bigger and more expensive and before you know, you will probably end up with something that looks and costs just like a normal jet fighter.
The F-35 is $200M. The Global Hawk drone is about $50M already and that's without any weapons systems.
