Because this system would show the attacker an account and password for every manager password entered.
With a normal password manager, the attacker is told if they're right or wrong. With something like this, when it tries Password! on the manager, it gets results, just they're wrong and it has to try to use one to find out. Then it tries Password1 on the manager, gets results, has to test, etc. So it has to add a verification step and not just attack the password manager. Of course, you're likely to lock out the account you're using for verification before you hit on the right password.