Are you saying that someone can hack a masternode if it's in wallet?
Any wallet can be hacked. It's easier to find if constantly online. A good passphrase should protect it, but a wallet that is 90% of the time in cold storage and has a good passphrase is way less at risk, even if the risk is small. But if you're like me, and everything you have that resembles money is tied up in your two masternodes, they mean the world to you, and you want the most protection possible. Some people don't like the hassle and feel the risk is so minimal that they run hot wallets. It's whatever or wherever you comfort zone is