A mixer can be used to hide or disguise the bitcoin footprint ?

Exactly what I was going to say, you could put the coins through a mixer multiple times in batches. It should clean them up, you could sell a bunch P2P on Localbitcoins & nobody would be any wiser.

Little correction, huobi never got hacked till now [1]. But, I'm not say houbi is 100% safest exchanges... we can't know what next about this exchanges.

Nope, the funds has been moved to 1M22U5uYEGUV1QkxSwUecebRGPW6uQgjRz and 38tgyi9ZobWNg5zUpzfDd1u5J3uTqyUdB2 [2]

Like anyone said above, bitcoin mixer will hide his identity and can't be traced.

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[1] https://www.blockchain.com/btc/address/14RYUUaMW1shoxCav4znEh64xnTtL3a2Ek
[2] https://davidgerard.co.uk/blockchain/2017/11/22/correction-huobi-wasnt-hacked-in-2015-but-the-2015-bitstamp-hacker-did-withdraw-12000-btc-from-huobi/

If there are no ways to use the funds why they will try to do so. Simple answer : hackers know the way to use the hacked funds , that's why they hack any centralized exchange try to do double spend attack on any blockchains. As @Mbitr said, they could use mixer to hide their identity. Also there might be a way to use the funds without converting to fiat currency which we don't know yet except the hacker community, IMO. If anyone want to know why they hack ,than he/she will need to think like hacker not like the hunter of a hacker. 

No such thing. There are lots of people who monitor the addresses of known and significant hacks. It's no work involved in setting up an alert.

We still hear about coins moving years down the line. A thief would have to do something to hide the coins. Simply waiting and then moving them in the open would result in action against them waking up. I expect law enforcement would be doing the same and exchanges too.

336 BTC isn't that huge. Thieves will mix them many-many times, with various methods, then they might start moving some of the coins between services to make them look more clean, then they might launder them or find some way to cash them out without major exchanges. They could look for OTC trading, p2p trading, exchange services, ATMs with cash withdrawals, etc.

If they are smart enough to steal those huge amounts, they are probably also smart enough to hide their tracks too.

I'd bet that they'll find a way to cash them out.

Just to answer your question it's because want easy money and stealing Bitcoin is the best way to do it. However, you said they can't use it but they can. They will usually sell the bitcoins easily after mixing the Bitcoins using a Bitcoin mixer service in order to make the transactions untraceable. Now imagine they use the mixer for multiple times. None can track then that easily. That's why, some exchanges and mostly local exchanges on clearnet won't accept mixed Bitcoins.

And yet many of them are so lazy and stupid they often do hang themselves. It only takes one slip to blow your anonymity.

The address that was posted on all of the hacked Twitter accounts has had past transactions to exchanges. The chances are there's some link to ID somewhere along the way.

Quite a few dark market druggists have sent funds directly from sales to exchange accounts that sent the pigs straight to their door.

maybe this is the point where most people hate banks (without much sense in my opinion) if they are convinced that the cryptocurrency system is also very broken in this part, hackers know that stealing cryptocurrencies is something much more advantageous than stealing fiat in the bank where you have security cameras and always have a witness and even if it takes many years, eventually the thief will be found, while in the case of stealing any exchange there is little and no chance of being found. That is why I believe that anyone who has money on exchanges (just like me) should not sleep soundly ... this is the great disadvantage of the decentralized world of cryptocurrencies.

But of course with KYC on all exchanges it might be possible to have a more secure system capable of finding hackers, but there are mixers that, although the intent of the mixers' creators are good (I think) help the hackers in some way and escape with stolen bitcoins