There are a lot of technical reasons. At the wholesale level there is no way to know if a source is valid. Even AT&T routes to dozens of providers for some calls and has no idea what is going where. Even local calls from someone in your own city might end up on someone else's VoIP network to get connected.

It is true though if the FCC started handing down fines that VoIP providers would require insurance/bonding all the way up. That would not fix it all but would be a large jump forward.

We also need end-users to force the issue. File suits and complaints. Every time we'd receive a complaint the resolution was "we blocked that number" and "we have no further info". If there was someone pushing a lawsuit through then suddenly everyone up the supplier chain would hand over their customer info right to the real end-user.

Instead, the FCC runs silly programs like their Anti-Robo-Dialer thing.