most people playing games like Civilization want an AI that they'll have fun defeating, not an AI that actually plays optimally.

The popular AI mods for Civ actually tend to make the AIs less thematic - they're less likely to be nice to you just because of a thousand year harmonious and profitable peace, for example, and more likely to build unattractive but efficient Stacks of Doom. Of course there are selection effects on who installs such mods.