Because an unmanned drone and an autonomous drone are not the same thing. An autonomous drone is always unmanned but an unmanned drone is not always autonomous. Autonomy means the ability of a drone to make its own decisions and calculations based on information about speed, location, altitude, targets, surroundings, terrain, and enemy aircraft in its proximity and to act without human input. An autonomous drone can be programmed to take an action based on a specific trigger such as coming within a certain distance of an enemy plane or alter its GPS course. An unmanned drone might need human input from the ground to make the same decisions that an autonomous drone could do by itself. Reason why unmanned drones are vulnerable compared to fully autonomous drones is Iran was able to hack a US RQ-170 and land it by forcing it into autopilot mode. That drone relied too heavily on ground systems and Iran exploited that vulnerability by spoofing its GPS. A different autonomous drone might not have relied that heavily on ground towers and would have used its own internal GPS instead of ground GPS that could be intercepted and spoofed. In other words, it is harder to make a drone that can take decisions without a ground crew but such a drone is also less vulnerable than one that needs human input. Any UAV that needs too much ground input needs communication for it and excessive communication is a vulnerability waiting to be exploited by electronic interference. Keeping as many systems of a drone inside its own computer and not sending them all to the ground and then beamed back to the drone is a good thing. That way there is less that can be jammed or disrupted between them.
