not everyone, no, but enough people since enough iteration of the game to make the question relevant.
that's the human's player objective. And not even everyone of them, I've never ever played any game of Civ to achieve one of the defined victory conditions, I've always played to stand the test of time, nothing else.
The goal of an AI in a game is to allows the human players to play an interesting game. That means it should be able to win of course, because challenge is one of the things that will make the game interesting, but it also means that it may have to be specifically programmed to not use frustrating mechanisms against the humans players. And I'm not even talking about role playing here, just "playing".
In short the AI is programmed* to play civ6 against humans players, not to play civ6 against itself.
*edit : "should be programmed", because IMO it fails at that too.
The question, as posed, presupposes that there is a problem without considering
the many ways to play Civ, or player expectations, or the time spent on playing
the game. Written in the style of a header for an article in a cheap games
"magazine", it assumes that there is a "malaise".
If some players exhaust the replayability of the game, because it is boring or
too predictable for them personally, then the real issue is how long it took to
reach that state.
It will take complete novices many, probably dozens of hours to learn Civ's
intricacies; to get to a level of competence where they recognise certain
patterns in the late game, is going to take dozens more. Just completing one
complete cycle of civilizations, game speeds (that suit) and maps will take
dozens of hours more. If they don't like the game after a few attempts, then
that's just tough luck for them - Civ isn't ever going to suit all possible
players.
Experienced players, who are the most likely ones to find problems with some
stages or aspects of the game, know that there are mods available to tailor
the game to better suit their style and preference. They are also the same
players who would have known what to expect from their experience with previous
Civ releases, and how long it took Firaxis to get them to a high standard.
Firaxis deserve strong criticism for many aspects of Civ, but not providing a
game that's value for money for the vast majority of players isn't one of them.
They didn't promise a game that is replayable with the same level of enjoyment
for infinite repetitions. Mods go a long way to extending the life of the game
and making it enjoyable for longer (for many people). That's one reason they are
a feature of Civ. I agree they are well behind "schedule" (if that's the right
term) to make good on their promise of better modding tools. If players don't
want to avail themselves of the many mods available now then that's just their
bad luck, and maybe it's time for them to move onto some other game. There are
plenty of other games around to find fault with. ;P
That's literally not what I said, unless you accept "playing to win" and "playing competently" to be the same, which I do not. The truth is that you can play the game very well--perhaps not "optimally," whatever that means--but well without single-mindedly pursuing a victory condition.
And there's just oldtimer "bumbling".
At marathon pace against 25-30 civs on ludicrous-size maps I often get
distracted by a series of events (sometimes fortunate, othertimes more Lemony
Snickett flavoured) requiring immediate attention. Those events and
interventions can then create other distractions all over the map and before
long (e.g 200 turns) I forget what the hell I'm supposed to be doing.
Kids - you can't do drugs and always competently run a world-wide empire!