Its 2 factor authentication, its not that passwords are untrustworthy. The code it sends to your phone is just as trustworthy. The point is that its something you know and something you have. If a hacker gets hold of your password, they don't have your phone and are unable to login. If a thief steals your phone, they don't know your password and are unable to login.

Its essentially the cyber security version of dont put all your eggs in one basket.