Twitter already has a huge infrastructure for SMS, reusing it makes some sense from an internal-logic standpoint.. For Apple's use-case, their users already ha

Twitter already has a huge infrastructure for SMS, reusing it makes some sense from an internal-logic standpoint.. For Apple's use-case, their users already have phones, and understanding TOTP would make it harder. Push notifications are already pretty straight forward, so building on top of that also makes sense. For Google, for instance, I often use their SMS verification rather than going to the authenticator anyway - It's faster, easier, and gets me logged in. It still raises the barrier of entry versus just guessing my password.