Consensus & availability are the two reasons.  People want a defacto way to verbally connect to someone and write it on their skin or a napkin and they can assume someone will have that and it can be used to contact them using several form factors. There's also the fact that there's infrastructure to communicate without a smartphone our computer available. Institutions require that people have them to be successful. Job interviews, official business, etc. Not everyone has the privilege of interacting in a digital only world. A skilled tradesman would have constant trouble interfacing with their customers. If a decentralized alternative worked through the phone lines, it would be pretty sweet, or there existed a way to take over control of phone numbers from those entities.  I wish we could make them "handles" for another system like keys or dids.