I mean, you’re not wrong, just a hair off. It’s the most universally possible to implement.

Every version of every phone can support SMS, and no one worries that someone is spying on them when they get one.

SMS is a terrible solution, but it’s extremely easy to implement, and very accepted by people at large. That makes it all those things you mentioned, but it’s backed by a very legitimate motivation.

In other contexts this explains part of the popularity of federated signin systems, since users may not trust you, but they probably trust their email provider, and if you can piggyback off their MFA, you don’t have to hope the user will find you special enough to do the extra work.

Dedicated phone apps have a similar advantage, since you can leverage the phones built-in identity management.

Passkeys are currently being pushed very hard by security folks because, if done right, you can make the user more secure while making their sign-in process simpler, and letting them need to remember less and not install or manage anything.

You still have the ultimate issue of the atypical user who is valid and can authenticate, but for whatever reason has decided to only posess the dumbest of dumb phones, and can only accept SMS or phone calls.