[bunderbunder]

No one thing discredits all of them, and personally I think that Occam's razor compels us to agree that in the vast majority of reports people are describing an actual phenomenon they saw.

However, the sheer variability of these reports does discredit the idea that there is a common explanation for all of these sightings. And that's where things like the alien visitation hypothesis begin to unravel. For the most part UFOlogists fail to see the forest for the trees - they look at each story individually, and never really spend any time trying to figure out how they could all possibly fit together into a single coherent story. Occam's razor also compels us to seek different explanations for different phenomena in the absence of any compelling evidence to show that they are, in fact, related.

The problem is, it's nearly impossible to come up with explanations for every single isolated anecdote by virtue of that very isolation. Skeptics understand this truism so implicitly that they oftentimes even fail to realize it's worth mentioning.