A knife is physical evidence
If the knife is produced with say, the victims blood and the suspects finger prints, couples with the eyewitness testimony then case solved,
If someone (claimed eyewitness) says, without physical evidence "i saw the man standing in the dock knife the victim" then that testimony cannot be relied on, is open to deep scrutiny as to the character of the witness, whether there is ulterior motive, mental state of the witness, whether the claim fits the facts, etc. In the majority of cases, only if there is corroborating evidence can the eyewitness testimony be accepted otherwise it will not influence their verdict. Of course some backward judges may still accept eyewitness testimony, only for the verdict to overturned on appeal.
In the past it has been accepted, see my links to see what happens in the case conviction by eyewitness testimony alone.
And see you are still stomping your foot in incredulity but consistently continue failing to supply your claimed evidence.