Because if the user chooses a bad passphrase (hint: he will) then his key will be compromised forever.
Users will choose 'password'; they'll choose '12345'; they won't choose 'VPSJjNwytizY7vtGzRv208' or 'cZD2N49caaSYkDuWZMHuyiuKuk6AhA5jdC29g3moHmX' (128- and 256-bit, respectively).
Users will choose 'password'; they'll choose '12345'; they won't choose 'VPSJjNwytizY7vtGzRv208' or 'cZD2N49caaSYkDuWZMHuyiuKuk6AhA5jdC29g3moHmX' (128- and 256-bit, respectively).