(Noob Question) Why do I need a hardware wallet instead of trusting DEX?

[LiveAwake]

Hi all, I'm new to Bitshares, so forgive my ignorance here. 

When I first started reading an overview, and the concept of the DEX (decentralized Exchange), it seemed to me like the DEX would offer a decent level of security over having funds on a centralized exchange.  If that is the case, do I still need to have a hardware wallet or some other method of keeping my various crypto-assets safe, or can I just transfer everything to the Bitshares exchange and feel fairly secure that they cannot be hacked there? 

Thanks in advance!