It would be less expensive to simply trust everyone. Administering a numbering system and trying to prevent fraud costs more than the actual fraud it prevents, and does nothing to prevent the larger frauds.
It's like having a chain on the pen at the bank, with a security guard watching the chain, and three managers making aure the secuirty guard is watching the chain all day, but the cash drawers are open and the three managers simply help themselves to as much cash as they like.
I was hoping for a decentralized version of discord ever since they started requiring phone numbers.