Agreed this occurs frequently and I agree with it: people should not lie under oath on a material fact.
It's like this:
An official launches an official investigation into whether someone committed child molestation. But it's clear, or at the least reasonably knowable from the outset, that the victim of the crime is NOT A CHILD.
(In this analogy, we are stipulating that child molestation is the only charge possible.) The investigation continues nonetheless and, in the course of the investigation, a witness lies about how he first learned the identity of the victim, who is NOT A CHILD.
So what material fact is this lie relevant to?
How does not learning exactly how the witness learned the identity of the victim impact the prosecutor's ability to determine that no crime was committed because the victim was not a child?
If the prosecutor knew or must have known going in that no crime had been committed (and there would be no crime if a necessary element was wholly incapable of being satisfied) then any indictment or prosecution, even a derivative indictment or prosecution for an offense such as obstruction of justice or perjury, would be subject to dismissal for abuse of process and prosecutorial misconduct. This would also give rise to a civil claim for wrongful or malicious prosecution and perhaps a constitutional claim against the prosecutor personally for denying the accused of due process under color of law.