Because anything can be “debunked.” They 'debunked" the moon landings, as I recall. All that debunking does is shows that something can be faked, not that it is, or was. “Debunked” does not equal “invalid.”
There are enough phenomenon that occur that have yet to be adequately explained (i.e., demonstrably proven/disproven), that we can come to the conclusion that we don’t know everything there is to know. Therefore, when phenomenon present themselves they are worthy of investigation until we discover those causes, whatever they are.
That people are deluding themselves. Dowsing is something that can easily be tested, and has been found to be fallicious. But we’re not talking about dowsing here.
Communicating with the dead, however, has produced mixed results. Some obvious frauds have been exposed, and in other cases results have been produced which seem to indicate that obtaining information likely known only to a deceased person has occurred. (Note careful wording.) Just because there are dishonest mediums doesn’t invalidate all claims.
Hey, there’s crooked bankers, too. Does that mean that banking is fraudulant?
It’s also a difficult field of study; we don’t yet know enough to control the parameters or results. If we just ignore it, we’ll never know. Remember, all sciences began with observation first!
Maybe the premise is wrong, too. Maybe it’s not communicating with the dead, but accessing some preserved memories somehow. If we don’t ask questions we don’t get answers, and we remain ignorant. Some of our greatest discoveries have come about by people thinking “out of the box,” having an open mind and critical thinking skills.