As the article explains, SMS is just plain insecure. So attempting to rely on it for MFA is worse than nothing (worse, because it allows attackers to lock out legitimate user).

Ignoring security (not that we should), it's just a massive pain from a pragmatic point of view. As others note, right when an SMS MFA is triggered is most of the time when I can't receive an SMS, so everything breaks when I actually need access.

Much easier solutions exist today at more convenient adoption at scale, like email and TOTP.