If you know who the murderer is, why not just tell the cops and be done with it? Why invite the murderer to hang out in a room with lots of other people while you publicly accuse them of being the murderer? You may be thinking that it’s so that there will be witnesses, but I say that all those so-called witnesses are gathered together in one room for convenient kill-ability.
Furthermore, if you’re the murderer, and some detective invites you to a gathering where he or she promises to unmask the killer, why show up? Does every murderer walk into the room thinking, “If I don’t show up, it’ll be a dead giveaway?” Does it not occur to them that attending the gathering and having the detective point at them and say, “That’s the killer, right there,” will be a dead giveaway as well?
I suspect if a detective tried the old “You’re wondering why I’ve called you all here” gambit in real life, either nobody would show up, or the detective would find themselves alone in a room with the murderer, because none of the other suspects would want to attend a gathering where they know there will be at least one murderer.