That's not a technical reason. They could just as easily ask for cryptocurrency payment of $1 (which is more than a phone number would cost), or even proof of work. The fact that they require an identifier that is typically tied to one's real identity is suspicious on its own, IMHO.
Non-tech-savvy users or those that want to use their phone number as their primary identifier anyway could still "pay" with SMS verification; others could pay with money and pick their own identifier.
