With that kind of money involved, I'm surprised this doesn't happen often. Good thing that people are so honest.
Hacking a centralized exchange tends to involve either exploits of weak spots in their security or inside jobs. As others mentioned, it's not really about honesty because addresses are known, and also because an exchange has no motivation to cover up the loss because in this case they'll have to reimburse all of it to their users, whereas it's a fairly rare thing for exchanges to do. Given that exchanges aren't safe, and your money there isn't truly yours because you're not the only one who has access to it, it's usually advised not to keep big amounts of money there. I follow this rule, but many people keep hodling on exchanges in hopes that the worst won't happen to them.