...In theory, Firaxis could make an awesome and unbeatable AI opponent with the calculated ability to know which path is optimal and how best to handle a situation, but what would the fun be in that? What is more fun? Playing against the best chess AI, or playing against a human player who has consistently bested you?
This is patently untrue, for several reasons. First, the best chess AI can't run on a gaming rig, period. The bigger issue is that chess is a relatively closed system. Each turn, the AI can move at max 16 pieces (and usually less). Each piece has a possible 8ish moves on average, making a total move space of 16^8, or 4 billion possibilities. That sounds like a lot, but for a computer it is manageable. Chess AIs aren't smart at all - they barely qualify as AI. They brute-force compute the possible outcomes of all possible moves for ~15 turns, then pick the best. There is no strategy involved, just as many processor cycles as can fit in a standard chess turn.
Now let's take the example of a typical invasion in CiV. A decent army will have about 16 units - so far, so good. Each unit has 19 possible moves (we'll limit ourselves to 2-move units), making a move space for the army of 16^19, or 75 billion trillion moves (7.5E22 for all my homies in the sciences). Also, each move has a less certain outcome - in chess, the result is binary. The space is empty, or the move results in a kill. In CiV, units have hit points, and the combat is random. This means that subsequent moves in the same turn rely on what happens previously. So take that 75 billion trillion and multiply it by 16, because you have to repeat it for all the units. And, the units don't come for free at the start of the game. There are cities which produce and spend resources which must be balanced between military, growth, and gold. All of these things are open ended and complex, with multiple interacting factors. In short, CiV AI is bad exactly because CiV is the kind of simulation all AI, real or theoretical, are bad at.
Of course the AI could be better, and honestly it should be better - it's been the weakest point of the series since the beginning, and it arguably hasn't seen drastic improvement since Civ 2 (although it has kept up with the myriad new features that have been added since then, for the most part). But for you to say it is easy to implement trivializes decades of AI research.