Why are we using letters?

Are we avoiding leaving RF spectrum traces? Are we worried about compromised digital channels? What is the reasoning?

In 2026 I'd rather be fighting for the army that evaluates all options to come up with the most effective way to accomplish an objective rather than one that dogmatically clings to ineffective methods.

In other words: if the winning side uses letters and the losing side uses phones, I'd rather be on the letter writer's side.