I was reading about Cashaa, that was hacked and 336BTC were stolen. But the hacker can't move them right? He will leave footprints. So why should hackers steal them since they can be found? For example if they want to convert them to fiat.
The only way I think you could get by from this felony is to keep them on very long term. So long that it may be forgotten. But still, it may.
There's a lot of ways, bitcoin mixer is one. And if you are asking why do they steal btc, its because they know ways so that the stolen btc will not be tracked. They know how they will not be caught. They are smart for hacking huge amount.
Isn't it obvious too that a thief steals because he needs money to be rich as easy as they can. I'm sure they also have a plan for that stolen btc. Who knows what is it, same with their intention in stealing btc's.