The point isn't to authenticate control of an account, it's to tie the account to some kind of expensive-to-replicate real-world cost, ideally one that most potential customers are already paying for.
Phone numbers are nice because the marginal cost to a customer is low (they probably already have one) while the marginal cost to a bad actor is high (it's expensive to acquire many of them or to change one once it's been identified as malicious).
Phone numbers are nice because the marginal cost to a customer is low (they probably already have one) while the marginal cost to a bad actor is high (it's expensive to acquire many of them or to change one once it's been identified as malicious).