There is no objective reason to do that. What's more, you want them to act as competitive human players, which is defining or limiting the game further.
They are representative agents in the same slot as the player with the same victory conditions available as the player would have using that nation. The game defines them as players per its rules.
I beg to differ. They are rules of the game.
No, I brand them what they are. There is no rule stating any player has to care about warmongering for example. I can, as a player, even choose to ally somebody over it and the game lets me. That is conceptually distinct from "killing x unit has y consequence consistently" in an objective sense, same as warriors having two movement points.
Sending an embassy to get a diplomatic bonus is an element of the game, that makes only sense because there's a rule saying it boosts relations.
You get other information with an embassy, including some that would be relevant to some situations in competitive MP (IE who is trading with other nations).
If the relations had an impact on the human players (e.g. can't declare war on an opponent if your relations are positive), you would probably brand it a rule too.
Yes, with good design mechanics would have reasonable incentive to use depending on context even with player vs player interactions. That Firaxis doesn't tend to get that right consistently in diplomacy is an issue.
But it is the only example Firaxis gave us.
Demonstrating that is isn't a viable example, and with pretty clear evidence (align civ 5 AI behaviors vs competitive MP behaviors), it should be obvious that I am pointing out that there is not "one" example. There is no example whatsoever.
Firaxis has never made an AI that plays to win, and the claim that it ever did was bogus. Based on the evidence presented in-game, I'd be willing to say that directly to a developer if they wish to challenge this particular statement. I don't think even they could make a rational, self-consistent case that the civ 5 AI consistently played to win.
Again, please consider jsut the diplomatic side of the AI. Civ V and VI inability to win a war or game is just incompetency and not related to whether or not it tries to win.
If the "diplomatic side of the AI" needs to be different from MP diplomacy, the design of the game's diplomatic system is flawed. Players frequently cite wanting to have meaningful interactions with other civs in the game, then complain when the AI doesn't manage it.
The fact of the matter is that problem stems from a bad diplomacy design, even more so than bad AI. The incentives aren't there, and asking players to play pretend like the incentives do exist is disingenuous. If it weren't for heavy UI regressions over the past two civ games I'd probably consider the diplomacy design among the game's larger flaws.