As a purely practical matter, try counting the number of unique pairs of N countries. It's N(N-1)/2. With N = 28, you would need 378 separate agreements. A nigh

As a purely practical matter, try counting the number of unique pairs of N countries. It's N(N-1)/2. With N = 28, you would need 378 separate agreements. A nightmare to keep track of. That's one big reason why we have things like the WTO etc., to replace a jungle of different rule sets with a single set of them.