Also they do measure position to achieve their feedback, might as well just output that on a 4th wire.
If you think about it, it actually makes a tiny bit of sense. First, it is failsafe: Breaking the control line or shorting it to ground will not move the servo to 0%, shorting it to signal level will not move it to 100% - it just doesn't move at all and stops applying force. Any sentient being within the movement range will definitely prefer it that way instead of random movements. Second, it can actually be pretty precise: The driver circuit can be completely analog, it doesn't have to be limited by arbitrary digital quantization steps. All it needs to do is check if the current encoder value is above or below the target and apply power to the motor accordingly.
So when you use hobby servos for robots, you're taking a low precision actuator meant to make a flap or throttle or other control surface go relatively up, down, in, or out, that was designed in like, the 70s, and asking it to do modern robotics stuff