Even if they have high security level, they are still vulnerable to potential attacks. Softwares nowadays are getting sophisticated so it is only a matter of time when those hackers will use the appropriate tools to attack their walls. So if you are a crypto user, never store your coins in exchanges for long term. Better store it in your own wallets.
That's good advice, but cold wallets are vulnerable, too.
A high level of security is appreciated, but break-ins are not excluded, it is important to react extremely quickly to the threat and eliminate it as soon as possible.