The same can be said for your home. Passwords are just locks on doors.

Until a better system or method can be implemented with the same level of convenience as usernames and passwords, we are stuck with them. In the meantime, we use them to stop most crimes of opportunity. But our sense of security that allows us to function normally in this world still requires us to trust each other to a certain degree. Your only other option is to give in to fear and paranoia.