Why is this? Well, if you choose to "use rubber hose cryptography" on someone to steal their Bitcoins, you've graduated from a faceless, online, white collar crime that many police departments won't even take a report for ("they've stolen your Bitwhats? Yeah, we'll get back to you.") to an extremely serious crime that will get you locked up in violent prisons for a very long amount of time in every country of the world.
So it's not surprising that the scenario you describe has not happened. It may happen yet, but it will be a rarity and not a statistically-meaningful threat.