Not just "normally", but simply an inevitable consequence of how the circuit operates. (You might imagine an inverted version where the positive rail would instead be commoned. Has anyone seen one?)
The point is however that the control line to the servo must travel along with the ground, so in the OP's diagram above, each control line goes back along that negative line to the LiPo battery, then along with the negative alongside each buck converter and from there along with the regulated power to its respective servo.
And PWM pins are generally not relevant to servo control.