Until now, the wizarding world had yet to figure out why the "Death Finger" spell could result in instant death. Perhaps some powerful individuals had figured it out, but chose not to publicize their findings.
The emergence of liches was a relatively recent event, and this spell was even more so. Perhaps, after some time, the principle behind it would be thoroughly researched.
The common understanding was that the Death Finger was neither a physical attack nor a mental attack, but some sort of rule-based attack. The wizard who invented this spell had unintentionally mastered this rule but knew the phenomenon without understanding the underlying principle. And this rule might be related to the legendary Underworld.
Generally speaking, powers involving the rule level were extremely profound, unreachable even by primordial soul wizards. Thus, the Death Finger was an anomaly.
In fact, from the current practical effects observed,