Civ 3 is the only other Civ game I've played, so I don't have much to compare in terms of aggression, but the AI wasn't very good at fighting wars in C3C. It tried to make up for it with unit volume (build & support bonuses), but it wasn't that hard to bait the AI into making bad moves.
Just to be clear, I think Civ 6, has the most complex and advanced AI of all Civ games.
In Civ, before the one unit per tile limit, siege mechanics, and stuff all the like. The AI was really not smart, it did not need to be smart to challenge a human player, cause they could actually dominate the world and sometimes they did, within the simplistic game rules of the time.
Current game rules make taking cities extremely difficult, and the AI lacks the tactics to do so effectively, and in addition it is programmed to not even try very often. As a result it feels very pasive.
In addition the AI is trapped in a very shallow diplomacy system. As this is the most important way humans interact with it. The problems with the diplomacy are perceived as problems of the AI. The diplomacy does not allow for active complex behaviours that propel the game, so this contributes to that passive feeling.
Finally there is also the hypercomplex city building optimization mechanics that is not very compatible with an general style that tryes to play balancing all strategies, which is what the AI is programmed to do.
So in summary I think all the problems in the game are interconnected make each other worse. All contributing to make the AI enemy civs lack pressence.
This would be my diagnosis for the perceived lack of competition, agency and agression in the AI, and that feeling of them just waiting for the player to act against it.