LOL! Easy? That is one word I've never heard applied to AI in a strategy game. It does start with good models and systems in the design and on spreadsheets, but that's just the beginning. If Sid Meier has a difficult time making diplomacy work in a game, I can't imagine that it's particularly easy. He's got some pretty serious grog credentials. Paradox has some brilliant people who are grogs, and it's not easy for them, either.
The real limitation to something like diplomacy tends to be performance. If 50+ nations have to calculate how to react to many possible diplomatic options, the amount of calculations can quickly spin up to where it's really borking your performance, especially a few hundred turns in, when hundreds of units are fighting each other all over the world. .
I think Sooner's friend is correct, the root problem is the core system. But you don't make computer games all at once. Often a flaw is only apparent when a lot of the system is already in. You can restart but it's always a challenge at the best of times to finish a game before you run out of money. Good engineers and artists are not cheap. And in this genre, it's unlikely we're going to sell 6 million copies.
I reckon it's a problem with the interface between the game mechanics and the AI. If you can have an interface that offers the AI a simplified representation of the
relevant aspects of the game world, along with the capability of 'simulating' the outcomes of diplomatic actions on that simplified representation, then you can apply a huge number of algorithms and techniques derived from the idea of Monte Carlo Tree Search
http://link.springer.com/chapter/10.1007%2F978-3-642-12993-3_3If you can apply those techniques to a game like Cataan, you can apply it to pretty much any Civ-like game. It wouldn't be hard to devise a way to map the state of a Civ-like game, into the state of a Cataan-like game. The thing is that, in order to apply this concept, one needs to define the architecture of the game in a very particular way from day zero. Retrofitting these interfaces on an existing code base can prove quite difficult.
Regarding performance: I'd say that a turn-based game can be harder, since all decisions (and plays) are made (resolved) in a sequential manner. In a WEGO game, the decision making can happen in parallel (and the resolution of the selected actions is usually *much* faster) making usage of multi-core architectures a no-brainer. In a real-time game, where you basically break down the notion of time into a 'infinite' sequence of time slots, and each slot is 'filled' with some events which are posted by the AI (or by the player, when he issues an action request from the UI), you can schedule the 'AI decision making' events in a way that suits your multi-core architecture best (so the evaluation of the actions is asynchronous). The EU 4 engine exploits this to great advantage.
Not to mention that you don't need *optimal* AI play to provide 90% of the players with a more than decent challenge. You can always limit the 'depth' - how far into the future the AI projects the outcomes of its actions - of the reasoning to your liking, as well as the 'width' of this reasoning (by avoiding to consider all possible outcomes, and restrict either to 'good' outcomes or 'sample' these outcomes on the basis of a time budget).
Listening to Soren Jonsson on 3MA - which is credited with the AI in Civ 4 - I can't help thinking that if he represents the 'state-of-the-art', then the art is indeed quite rudimentary.
Plus, no matter how successful you are, one multiplayer game will blow away the best AI. Nothing beats playing against other meatbrains.
Every year that goes by, the dominance of the 'meatbrains' is contested in a different game. I'm not a friend of Kurzweil like pipe dreams, but I think we overrate ourselves quite often. The 99% of chess players in the world can't even dream of competitive play with the top 1%. If you can write a program that can offer an interesting challenge to that 1%, is probably going to be quite challenging for the 99% as well.