Radios are all well and good - but a busy frequency malfunctioning box/aircrew or any distraction could lead to your No.1 steaming off without you.
The main principle of formation is looking at each other - it's no.1's job to keep everybody else in - from the word 'go' to landing. Keeping eyes on is the main principle of everything. Pilots even learn how to communicate with gestures to explain they're out of fuel, have a system failure or want to slow down, level off or turn.
Singletons on carrier catapults are doing the same thing with the deck crew - everyone needs to work together and more importantly communicate effectiely. Big, deliberate gestures is the way it's done.