Yeah you would be fine at six feet too. But if that person just stopped moving to check their phone, turned around and doubled back my direction and passing me on the narrow sidewalk, all kinds of reasons that they would quickly close in from a safe distance to within six feet of me, that wouldn't be too great. Easier to just throw on the mask sooner than wait till the last minute.

But packing tens of thousands of people elbow to elbow outside while wearing a mask is perfectly fine.