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.
I don't think they would ever be able to hold it for long that it would just be simply forgotten. Usually these hacks affect a large number of people, and many of these will check regularly to see if the funds have been moved. Even if the number of people checking does dwindle over time, there will still be the odd guy that checks every year or so, just in case (someone just like me).
I was scammed for 14 BTC back in 2016 and the guy still hasn't moved the coins. I still check the address on etherscan every few months, just in case.
Also, it's not that hard to cash out illegally obtained Bitcoin's, there's a few wallets like Wasabi and Samourai that feature built-in fungibility and mixing tools. Though these might be breakable if somebody has enough data about the participants involved in a mixing session.