It's just not practical. The number of layers a game like Civ has, with resources, terrain, technology, units, trade, reputation, culture, wonders, improvements, city buildings, specialist citizens, great people... how the fuck do you make something that combines all of those factors each turn to choose the best path? Worst of all, probably the most important single variable in each match is predicting how your opponent will behave, which means the computer would need to learn what the player's habits are. At that point you've basically created true AI. And considering how long it takes to resolve a late game turn of Civ as is... yeah.
Of course, the way humans play these games (or do anything involving significant complexity) isn't by simultaneously cross referencing all the factors. We form a hierarchy of strategies within strategies.
- I started near metals and can use production better with units than building so I will be aggressive this game.
- I'm being aggressive this game so I will build archers instead of spearmen, and will attack my nearest neighbour.
- I'm at war so I will pick war technologies to pursue, and ignore culture and trade
and so forth.
You don't pick what technologies to pursue based on what resources you have (or maybe you do, but then you don't decide to be aggressive on that basis, but on the basis of your technology. The point is that it works like a flowchart rather than some sort of weighted equation of all variables at once.)
As far as impossible difficulty goes... I'd assume a sufficiently brutal combination of options on AI Wars would be impossible, or close enough.