The reason it's not enough is because (a) there's also the middle ground of being mistaken, for one reason or another, so even if you were able to rule out downright lying then then that still leaves a second option either than telling the truth; and (b) since lying is actually something that some people do occasionally, I don't see how you can rule it out anyway, ridicule or otherwise. Maybe someone thinks (or hopes) the lie is convincing enough that they won't be found out; maybe it's just a different form of trolling; and goodness only knows how many people have lied about having psychic abilities because (surprise, surprise) they're able to sell their stories and/or their "skills" to make money.
None of the cynical reasons for lying apply to fender, I'm sure, but -- I mean, use your imagination, Naomi! Even with lying being ruled out, there are myriad possibilities between "people who tell stories about ghosts are always lying" and "ghosts are real", at least some of which could be just as plausible.
If it comes to that, why would psychologists lie about their observations? Or, if they don't, and are simply "wrong"... well, you can, I hope, see what I mean about the double standards that are at play here.