Keeping your money on an exchange certainly does make you an easier target, but some people simply do not have the ability to store their cryptocurrency safely in any other way. They might not have access to safe storage along with a personal PC, but want to purchase and hold it. In theory an exchange can be much safer than a private PC which could be vulnerable to viruses or even hardware failure - this sort of redundancy will be built into the biggest exchanges who can have vast security teams covering many different aspects. While it is devastating for the 6,000 affected individuals, the fact that it is not a complete loss of millions of accounts and is fairly concentrated to the low thousands is somewhat commendable.