SMS is available as a fallback 2FA method for Apple ID.
I'd love to use Apple's keychain for credentials for convenience but it can quickly become the weakest link, when it should be the strongest.
This is different than "Reachable at" phone number which must be unique and is used for iMessage and Facetime, and if it's blank other people can only reach you via iCloud account email.
(It makes sense if you think about it, parents setting up iCloud accounts for their children's iPads who might not have their own phone).