Also note that these AI systems typically use neural networks so there is a problem with transparency and control. A neural network might be exceptional at making decisions but you don't know why it makes them and you might not necesssarily be able to tone them down - who wants to play against a supercomputer and lose everytime? Whereas the CIV AI is at least deterministically controlled and explainable.
You may not be able to easily tone them down, but you could always use earlier AIs of the reinforcement learning cycle that still make mistakes instead of the latest one that has ironed out many of those. Once you have a framework for a learning AI, you would use the first passable AI that manages to play the game without hurting itself as the Settler AI and then you would start letting it play against itself. After some time, you freeze one (or several) versions and that is your next AI level. You can continue this for increasingly harder AI levels.
On the other hand, TBS tends to favor the human player because there is no need for quick reflexes. Both sides can take all the time they need. So the quality of the strategies and tactics will be more important. And this is where humans will shine. Unless you are dealing with some machine learning super AI, most humans will always strategize better than a computer. And humans are really good at thinking outside the box and dealing with the exceptions like the case where normally X is the right strategy but in this particular instance, Y is actually the better strategy. Computers tend to just follow a rigid set of rules, if X do Y. They can't really think outside the box like humans can. So strategy games like civ where there is no time constraint, heavily favor the human player.
At this point, I don't think you can claim that humans are able to strategize better than an AI. Go is a lot about long term strategy, and as of last year, the AI can outplay any human. What is true, is that at the current state-of-the-art, humans are much faster at recognizing mistakes and learning from them. An AI can "think out of the box", but only if you allow it to make actions that seem to be a mistake at first. But this means, that it will make a mistake many times until it recognizes that this mistake doesn't lead anywhere and it should just stop doing it. And for a practical AI, there is a time constraint for the AI, which are the turn times. If you take 3 minutes to play a turn, you wouldn't want to wait 21 minutes until all AIs have made their turn.
My reasons why there is no good Civ6-AI:
- lack of investment: You would need a large team of good (expensive) AI experts
- computational complexity: Civ always had the complaint of long turn times, complicating the AI would almost certainly make it worse, especially on a wide range of hardware that players have
- age: When development on the Civ6 AI was started, there was no way to tell that neural networks would have that much success
- complex win conditions: makes it harder for an AI to judge whether a move will put it ahead
- political game: There are many factions in a Civ game, which makes game theory very complicated
- immersion: Players tend to react badly to an AI that is trying to win at all cost. They will be complaining when a Gandhi AI with +100 relations attacks you because it deems this the way to victory.
