Then nearly everything is arbitrary by the same standards.
Speed limits are arbitrary -- why 55 instead of 56?
"Normal temperature" of 98.6 is arbitrary -- it really isn't exactly 98.6, it's really a range of about 97-99.
When we talk about "healthy weight," "healthy blood pressure" -- again, these are "arbitrary" -- as they are not exact.
So yes -- I guess it's arbitrary, in the sense that EVERYTHING is arbitrary.
But, like a normal temperature of 98.6, and like speed limits of 30 in residential neighborhoods and 55-65 on highways...
Social distance of 6 feet apart is indeed based on known science and good policy. It's not a random number picked out of a hat.