[chrisnight]

Why is the defining feature of being human the property of having a phone number?

Spam is indeed a hard problem to solve, but the issuance of phone numbers is not designed to be used as human identification.

[vel0city]

Maybe they're not necessarily going for "all humans that exist everywhere under any circumstances" but instead "humans likely to have access to a phone number which can sometimes receive SMS."

Not every app needs to cater to every single human and potential use case on the planet.