It’s simple, reliable, and effective. Shortwave receivers can be made fairly compact. They’re also very prevalent in most countries (every ham transciever), so there’s nothing suspicious to pack. People find numbers stations interesting, so they are often streamed online. One time pads have their logistical shortcomings, but are still the best encryption possible. The OTP can be compromised in known, visible ways, where a phone has myriad invisible ways to be compromised.

You could probably cheat with the one time pad and use a book as a key, pick a pre determined starting point go diagonally down accross the page convert the letters to numbers and xor that against the message. It would be near enough to random and less conspicuous than a pad of random numbers when searched.