The rarity of that scenario is dictated by there rarely being a need for the security it offers. But that, in turn, is a function of our knowledge of cryptography, and may change over time. Who knows; perhaps someday we'll see something like what Vinge described in AFutD:

"Our main cargo is a one-time cryptographic pad. The source is Commercial Security at Sjandra Kei; the destination is the certificants' High colony. It was the usual arrangement: We're carrying a one-third xor of the pad. Independent shippers are carrying the others. At the destination, the three parts would be xor'd together. The result could supply a dozen worlds' crypto needs on the Net for ..."

It won‘t. Symmetric ciphers are holding up just fine against quantum computers.

Key exchanges would be annoying, but they are even more so for one-time pads.

But it will never come to even that: There are plenty quantum-safe asymmetric cryptosystems around.

I've made matlab scripts that pull true randomness out of the less-significant figures of the output of an audio microphone listening to static...it's not impossible.

If we were serious about it, sharing flash drives and even using couriers could make it work pretty well.