The problem (and quite possibly the reason you're looking for) is that, unlike say ISBNs for books or internet TLDs, post codes are used by ordinary members of the public of varying degrees of literacy and intelligence and with an overwhelming ignorance of the mechanics of how their own country's postal system works, never mind other countries'. It just isn't possible to get everyone to behave in the same way, even within a country.
Not really true.
A well designed system should not expect users to know intricacies of internal systems.
Designing such system is not difficult.
In late 70s, the International Trunk Subscriber Dialling was standardised by ITU! Telecom version of UPU.
Each country was assigned a code, after seeing how many telephones were expected to work in future in that country.
Total code ( after international dialling prefix ) was fixed as 10 digits.
Countries with large number of phones were given shorter code and with less number of telephones, given longer codes.
Thus USA got 1, UK got 44, India 91, Armenia 374 etc.
Countries were expected to design their own codes within balance digits.
USA went for for Local Access Area codes of three digits followed with balance six digits.
Most of the other countries followed a city or county wise pattern with big cities getting smaller codes and smaller cities getting longer codes.
The arrangement got modified with coming of mobile phones where most countries changed their internal system of coding. The international system remained same except that total size of code+ number was increased to 12 digits.
The system has worked fine. Any person, literate or not, only has to know the number and can make a telephone call.
In international post also, the tracking system was so designed. A tracking number is given but the data is not fed properly. For example, Spain postal or gain station does not feed the weight of the packet but Russian postal authority does. The Indian postal authority does not feed the next point of handing over so packet becomes untraceable. It is the sheer laziness and incompetence that the system was not designed to ensure discipline amongst workers.
In telephone networks, the signalling system was forced to be changed to a newer system of multi frequency signalling rather than pulse signalling. All countries, irrespective of whatever antiquated system they had in switching their own network, were forced to go for new equipment at their international interface. Thus countries like Albania, Cuba changed their system so as to generate and accept international traffic.