The game tree of Civ is vastly larger than any board or card game. In a single turn you make dozens of decisions about what to build, where to move units, who to declare war/make peace with, what technology to research… and on top of that, games last for hundreds and hundreds of turns on a map with thousands of tiles which must be explored, resources uncovered, terrain improved.

Humans are decent at Civ exactly because we play in a naturally heuristic fashion and don’t try to do any tree search. To get an AI to cope with the this vast amount of information, available and hidden, and make sensible decisions without tree search, is a huge challenge. On top of that, it needs to deal with up to a dozen opponents, multiple of which may be human, and avoid getting ruthlessly exploited in trade deals and diplomacy and all the rest.

Here is one basic decision among dozens made every turn: do I accept an open borders treaty with Bob of the Carthaginians or not? I can’t see any armies he may or may not have (fog of war) and he’s never betrayed me before so I guess he’s trustworthy… oops! He used the open borders to move a vast army into my territory and right up to all my cities in a single turn and then declared war!

That’s one decision, among dozens made every turn, that led to total defeat.