>For these reasons, I cannot figure out why they don't know who stole the MtGox bitcoins.

When you don't understand something, something might be wrong with your model. (http://lesswrong.com/lw/if/your_strength_as_a_rationalist/ comes to mind).

The exact addresses of the Gox stolen coins aren't known AFAIK. Even if they were, to cash out you only need to get someone to accept them without verifying ID. I can send coins to an exchange, and have them send me other coins in a different cryptocurrency, and send those to another exchange, then convert back to btc. If the intermediate currency is something like Monero, then the chain analysis must stop there.

I'm not saying most mixing happens through alts, but it is a fairly foolproof method for anonymity, at the cost of not supporting volume and high fees.

Also, you can sell to people for cash, so any investigation hits a dead end. There are probably dozens more ways to cash out anonymously.