There's not some magic involved where the virus gives up after exactly 6 feet.
6 feet was chosen as a matter of policy because it was the most you could reasonably get people to stay apart. Sidewalks, store aisles, elevators, etc, are all about 6 feet wide and so it's a good number to convince people to cross the street or wait for the aisle to clear. 100 ft would be a much safer number but that's just not really physically reasonable (and so the guidance would have been dismissed by the public).
And crucially, the context of the shared space makes a large difference. You simply cannot compare 6ft. outdoors with lots of ventilation vs. indoors and forced air vs. indoors and no ventilation.
The choir practice should not be an example of how easy it is to spread; rather, it should be an example of the unreasonable intuitions people have about 'distance'.
Remaining in close contact, indoors, while singing, for a long period of time is not a good idea. It is quite reasonable to expect that to lead to transmission.
Similarly, two people passing by each other on a sidewalk, with nose breathing and mouths closed, even if it's closer than 6 ft., is a very small risk. Still good to maximize space, but nothing to get worked up about.
Singing is also likely to expel more droplets further.
I don't have a source, but a microbiologist I know was saying that safe distances, based on droplet physics, depend on activity, e.g. standing in a single file line vs walking vs hiking vs running.
6 feet was chosen as a matter of policy because it was the most you could reasonably get people to stay apart. Sidewalks, store aisles, elevators, etc, are all about 6 feet wide and so it's a good number to convince people to cross the street or wait for the aisle to clear. 100 ft would be a much safer number but that's just not really physically reasonable (and so the guidance would have been dismissed by the public).