Games didn't sit for years - they were put out on route to make money. Games didn't sit in inventory at retailers, etc.. every few months the companies were shipping the next new game. If you sat on a game for 2 years, it was already a 'has been' and wouldn't earn as well as it could.. so there is no incentive for games to sit.

And yes, people knew alkaline batteries could leak - but you also find that the companies used good batteries that would not leak - like you saw here. Leaking batteries were often because after the initial batteries died, everyone replaced them with crappier batteries.

I can't think of any NIB opening of games from the 80s or 90s where the AA batteries had leaked.

You're trying to apply logic in hindsight.. like 'why would you make games unsafe to sit for many years?' - that was never their intent, so never any need to protect against that.