SMS is literally the least secure form of authentication, because numbers expire after mere weeks, and get re-assigned shortly, within months, because of number shortage in many area codes.

Nothing like this could happen with any mainstream mail service like Gmail, where it's officially advertised that the accounts could never be reused.

The worst part about SMS is that not only is there the potential to be locked out permanently, but also you never know whether or not the service would allow login or password reset via SMS, thus, you never know if you're opening yourself to account takeover.