Re: the latest iPhone hack. It's a serious question. I know you're going to say "Because they can, and to show that they're really cool and clever".
But Apple can update all iPhones remotely and there's certainly no shortage of functionality that they err... haven't got round to implementing yet. So with every update they're going to close the loopholes and exploits used by the current generation of hacks. And so the cycle will continue.
In the meantime, all the hackers have got to show for their efforts are a few bricked iPhones and lots of geek kudos and glory - maybe that's enough for them?
If you're a regular iPhone user, of course, just stay well away. Why risk bricking your iPhone for the chance to play with a few marginal third party applications for a few weeks until Apple closes the loop again?