The Good Friday Agreement didn't concern itself with customs and regulatory controls simply because it didn't need to. At the time the Republic of Ireland and the UK (including Northern Ireland) were already in a customs union and single market by virtue of their EU membership. Nobody at the time foresaw that this might ever change, so the issue of customs etc never arose in the lengthy negotiations preceding the agreement. Discussions on the border were concerned entirely with security and freedom of passage, not customs.
>> Nobody at the time foresaw that this might ever change, so <<
That’s absurd! The Maastricht treaty barely survived the House of Commons just a couple years earlier, only after securing an opt-out from adopting the Euro. EU membership was ALWAYS extremely unpopular in the UK. If the Irish signatories didn’t realize that, they were the biggest fools ever. If they knew that, and thought they could force the UK to stay in the EU by signing the treaty, then damn every last one of them to Hell.