At least for earlier ARR era codes, my theory is that they didnt know wether or not the game would e able to hold up even after their massive overhaul and didnt at the time forsee the massive success it would become. Call it a cautionary optimism.

With that narrative; I believe that codes with merchandise at that time were primarily to provide further momentum on their hopeful success and at the time wasn't seen as an extra to the product being sold, but as a topping to better cement their foothold on the comeback of their recent failure. In that sense, to me, I don't think they had the foresight to have codes that would last year's after those products released, because even though yet had the story planned for it, they couldn't be sure the game would survive for it.

That's what I believe anyways since I think more recent released material have codes that do not expire.

Though.... they did release some codes for the wrong game I think last month. So it could be incompetence too