What more do they need? Compelling testimony from an uncooperative witness isn't necessary for a case, nor is it even very helpful: such a witness can't be considered reliable or trustworthy.
In your world guilty parties walk because the state has no power to make anyone do anything. Everything is based upon the prosecutor being Sherlock Holmes and always finding exactly what they need. The real world doesn't work that way.
Are the italicized qualifiers intended to mean something other than that?