If the blockchain is public, why don't they catch the thiefs?

[shamntalk]

Total n00b question I know, but since the blockchain is public, and even the transaction from the MTGox theft is still displayed at blockchain.info, why didn't someone catch the thief?

So: thief takes money from wallet A to wallet B, then B to C, then C to D... all these are visible right?

Even if they are truncated after a while, isn't just a matter of 'listening' to the network (after all these transactions have to be accepted into the blockchain), then once the thief attempts to buy something or cashout, nab him?

This does puzzle me. 

[ufmace]

They're public, but also anonymous. It's designed to be difficult to tie a bitcoin address to a specific person or business. You can see all of the addresses that the stolen bitcoins went through, but proving that a specific person was behind the theft is hard. Especially if the thief takes a few precautions - sending through a few different anonymous wallet sites, etc.

[someguy123]

The coins will get mixed. That's why.
e.g. the bitcoinica thief came into a channel on IRC and gave away a large amount of the coins he had.
This resulted in wayyyyy too many people being a possible suspect, so he could then for example use blockchain's mixing service to make even more innocent people have those coins. After this has happened, there's no way of really knowing who the thief is.