Not only are people greedy in cryptocurrencies, but people in any investment industry are greedy. Don't trust anyone, not even your friends. Don't put all your money on exchanges.
Putting money or rather the better term is storing money on exchanges. There is always some people who will keep doing this and the people who do such things are not visiting this forum regularly.
Being greedy is common but bad luck can hit and exchanges getting hacked is pretty common. We have to reduce the amount of money kept there for regular transaction as much as possible.
Offline wallets are always safer as compared to online wallets.