It's pretty strong to assert that it is universally possible. From a technical perspective, yes, you can determine the origin of all VoIP calls in exactly the same way as you can determine the origin of all IP packets. This is to say, yes in theory, but in practice, no in many situations, not even necessarily when the origin has intentionally been obscured.
Cold-call spam may in many cases originate from corporate PBXs or VoIP setups which were compromised by an attacker. I'm not sure how common this is but know it happens as I have been involved in the response/cleanup on two different occasions. This is perhaps the most difficult to track case as it becomes a matter of forensics on the compromised PBX setup. This is less likely for inbound though because it's not a very stable arrangement. It is also not unheard of for non-VoIP telecom providers, generally overseas, to be themselves corrupt and involved in facilitating malicious uses of the telephone system (e.g. international calling termination rate scams).
On the other hand, there are a huge number of VoIP providers which may operate overseas, may not be very responsive to complaints and requests, and even unintentionally may not retain logs that allow them to identify customers. This means that when law enforcement determines that a call originated with a VoIP provider (via phone provide records), there is a high chance that getting information about that provider could require costly legal proceedings and even then result in a dead end.
I personally use a VoIP provider right now for completely non-malicious purposes that would probably be very difficult for law enforcement to identify me from because I know the provider to be overseas, small, and to have generally poor operational practices. This isn't even a provider which markets explicitly to criminal uses (which very much exist!), it's just an extremely cheap one.
Yes, law enforcement are certainly not doing everything which is possible. But it's far from a cop-out - law enforcement has limited resources and the way things are right now VoIP providers can easily become a very frustrating dead-end.