[bobthepanda]

- Every set of goods has its own regulations. Ships carry all sorts of goods.

- Every country has different customs regulations, so paperwork is not really that reusable across ports.

- Customs is a huge source of fraud. In poorly managed countries it is one of the prime vectors for bribery and corruption. So destination ports have to verify everything even if they do receive the paperwork, because there’s no trust.

Generally speaking if you want to cut customs red tape your options are some kind of free trade deal (which is not necessarily popular and never covers all types of goods), or banding together with a bunch of different countries to submit to one supranational entity like the EU. In fact we have a live example of how complicated customs arrangements can be; look at how trade from UK to EU has been disrupted by Brexit and the introduction of customs.