Actually its much easier to build fully automated planes than cars or tanks (because the sky is almost empty). But there are some major problems with drones that are controlled without "human in the loop". You need some degree of artifical intelligence to decide what to do. this is already feasible for "normal" flying like recon or cargoplanes. But when it comes to fighting / bombing it gets very complex and thus very dangerous.
Another problem is that it would be easy for humans to predict what such an AI would do next and so you could exploit its "weak spots" (e.g. if you wear civilian clothes -> a.i. will not bomb you).
That is why I belive that combatdrones will be controlled with human "in the loop". This means that the human machine interface is the weak spot (lag, jamming, range) and so you still need piloted aircraft.