Because the phone system is based on very old protocols with duct tape and patch work applied over the years. Not sure how you want VOIP calls to be de-anonymized. Are you arguing for state surveillance?
I used to work for a phone company. While this was not in the US I don't see a reason why it shouldn't be possible to implement this there as well.
Each call had two source number: One user provided and one network provided. The user provided number could be set by the caller (within limits). The network provided number was always set by the provider.
A normal callee would see the callers user provided number. The emergency services, however, would see the network provided number.
This worked for PSTN calls as well as for VoIP (via the P-Asserted-Identity header)
This can tell you the network that sent the call (with a lot of effort and subpoenas to every network along the way), but there are so many ways to get VOIP access these days, VOIP companies don't necessarily have KYC.
