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...
The parallel with telecoms doesn't really work. It doesn't matter what international agreements have been reached or how many digits phone numbers should have - if you dial wrong, you won't get through. You will have direct evidence of your error immediately and will either redial correctly or seek help if you don't understand what the problem is. Either way, apart from inconvenience and time, you probably won't experience material loss.
With the post, you don't know whether something's gone wrong until it doesn't get delivered, which in many cases is too late as the item has got irretrievably lost. There is no feedback on how you address your package at the point at which it leaves your hands, unlike with a phone call. The more detail you can give, even if it's not in the destination country's standard format, the more likely it is your letter will arrive as intended. As I mentioned above in the UK you can send to a house number and postcode, but if the latter is wrong even by one digit or letter, it will get irretrievably lost. Write the whole address in a way a human postal worker can understand, and a wrong postcode will be overridden by the rest of the address (admittedly possibly after a delay).
Furthermore, phone numbers are numeric only, and all technical systems in the world use the same number system even if for certain more literary uses non-western numerals are used in some places. Addresses are alphanumeric and are subject to linguistic and alphabetic ignorance on the part of senders and deliverers/sorters.