The one that were hacked was the exchange itself and not the bitcoin itself.
Security of the exchange must be improved but if a hacker focused too much on their goal and spend time on it, they will really have a chance to breached the system by any means even how secured and well maintained the security is. Again, "FOCUSED". Cryptocurrency is no doubt one of the most targeted now by the hackers for an obvious reason.
Why would people lazily leave their funds in exchanges in the first place anyways? What is the point of it? To avoid receiving fees for the deposit fees that are needed? It is entirely 50/50 decision that would lead to a 100% of something not probably gonna happen. I had left few tokens in some exchanges like EtherDelta, but they were just half a token in its composition and I just leave it as a dying investment anyway since I sold it all out when the token I was holding peaked.
Not literally leaving their funds but for a trader, it's really necessary to just leave their funds on an exchange so that they won't missed an action. They understand the risks here that's why it can be only applied to those big exchanges. I see nothing wrong about it.
Anyways Etherdelta is not a good example here.