Nowadays, it is difficult to rob actual dollars from a bank vault.
It is somehow easier to gimmick an electronic money transfer, but still there are safeguards.
Most coin exchanges are improvised, with the poster boy being "Mt Gox", which transitioned from selling Magic The Gathering Cards to exchanging BitCoins, without any thought being given to security or accountability.
As I mentioned before, mining coins is purposely difficult, with a mathematically proven difficulty level.
Devoting your computer resources and brainpower into hacking into coin exchanges has a much bigger potential payoff, albeit it has the inconvenient of being, well, illegal.
Others have been quite successful hacking into mining servers, so any coin successfully mined would be deposited in the "wrong" wallet, leaving the server operator wondering how come his $100,000 worth of specialized hardware didn't manage to compute a single BitCoin in 3 months of computing...
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