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
Actually we have been given the option to keep the wallet safer, i.e. with 2fa. as long as they don't have your cellphone then your wallet is safe. I almost had theft in my main wallet, and luckily I installed 2fa, otherwise it would be the biggest crime for me