Comment by sowbug

SMS has become a kind of real-world PoW (proof of work) mechanism. A phone number typically has a recurring fee to keep it working. So a live number indicates that someone is spending money (a proxy for effort) to maintain it.*

It still seems like a lot of money to spend on simple, old technology, but from the PoW perspective, making it cheaper would defeat its purpose.

*Which is why many sites reject Google Voice numbers, for example, for SMS verification.