It's because of the relative force required to articulate the main rotor blades and thus swash. It takes a fair amount of (relative) force to move the swash to get cyclic movements. Relatively speaking, it takes a lot of force to precisely articulate the swash to control the heli(s)

RC's use little electric servomotors that can only provide so much force. It requires less force to twist the paddles AOA. So for cyclic movement, the swash movement changes the AOA of the paddles instead of the AOA of the main rotor blades. This articulation of the paddles cause the flybar to teeter thus changing the main rotor AOA. The long leverage of the flybar does all the grunt work for cyclic. If you tried to create an electric servo that can provide the necessary force, it would be the size of a 1/4 scale servo (or more) and would probably need a large battery to drive it. Due to weight and size constraints ... this simply wouldn't be practical. So manipulating the main rotors via the flybar to harness aerodynamic power/leverage is used instead.

Real helis have things that literally provide much more force ... a human connected to a 2 to 3 foot lever (cyclic stick). This "human servo" connected to an adequate lever can provide plenty of force to manage the full size helis swash. Even with that, some helicopter controls are hydraulic assisted.

Human power, leverage, and hydraulic assist provide enough force to easily control the swash thus negating the necessity for other cyclic devices (flybar).

Good Luck,
d.tipton