In general, because you tend to have so much more entropy in a private key phrase (you only need one for multiple logins), and because the private key itself is

In general, because you tend to have so much more entropy in a private key phrase (you only need one for multiple logins), and because the private key itself is a nastier nut to crack, the computational effort needed to crack a private key file is just nastier than the password crack. That's the whole point of the design.