This is completely incorrect, just because the wallet is stored in your own device it does not mean it cannot be hacked, the reason we see more hacks against exchanges has to do with the fact that they are big targets because of the great amount of coins that they hold, but make no mistake there is always a way to get hacked or to make you to reveal that information, some months ago thieves stole bitcoin from a trader the old fashioned way and this is only going to happen more frequently as the value of bitcoin goes up.