Yeah some are not been a victim but for sure one day or another they will become one as well like what happen to other big exchange that ha a history of hackings.
sometimes it is the inside job that more thinks why this hacking come to reality though proof wasn't there to back up the claims but in some opinion this is indeed.
We still new to decentralized trading platforms, if we adopt more to decentralized things then we can reduce hacking.
Why? we do still end up on making transactions with fiat thats why people will prefer on using centralized platforms in the end of the day.About the question on which exchange havent been hacked yet
then i would say its Coinbase. So far this exchange doesnt have any issues when it comes to breaches or some sort.