Why are irrefutable mass sightings, like the 1994 Michigan case or the Phoenix lights not considered definitive proof of the phenomenon?

Given there are multiple well-documented mass sightings, why are they—if not individually, then at least when examined as a group—not considered proof positive of the phenomenon?

Finley park IL, 2004 O’Hare airport, 2006 Phoenix lights, 1997 Western Michigan, 1994 Hudson Valley, 1983 Westhall, Australia, 1966 DC, 1952 Battle for LA, 1942

The phoenix lights, western Michigan, and DC-52 flaps could almost be proof on their own. Any thoughts?