I wonder too about this kind of thing. But 6' matches my perception of reality. On a frosty day, how far can you blow and see your breath. If someone is standing in a sunbeam, watch the mist coming out of their mouth when they talk. 3 ft won't protect you. 6ft likely will. Being far enough apart that any moisture can fall to the ground has to be a good thing.

BTW, I don't think it was a politician who came up with it. More likely a medical recommendation. But I still wonder if it has been tested or if it is just a guess.