This is internet where anything is hackable.
Can we prevent it happen ?
Yes, the site for example MEW already tell you for security purpose, you should active 2FA, bookmark their website, etc
Everything in real life is able to be stolen as well, everything has a risk. Just because something is digital doesn't mean it has more risk. Its all just FUD from the mainstream bankers trying to bring bitcoin down, don't be swayed by their lives when we have a real chance to break free from their chains.