The most cynical answer is the correct one. They just come out and say it: "We routinely lie to defendants about what 'evidence' we have. If we record an interview, people will hear all the lies and the threats based on those lies."
- "we know what happened. we're just trying to fill in a few details."
- "look at this big folder with your name on it! all this is evidence against you! no, you can't see a single page of it."
- "and we've got your fingerprints, and all kinds of forensic evidence, and there's video of you, and your buddy already rolled on you..."
- "and we'll charge you with this, and this, and this, and this, and this..."
Looks like you have read about interview techniques.
Those are ones I have read about.
Reid Interrogation Techniques.