Because it is the only Nash Equilibrium of the game.
If everyone plays for Dom victory, you cannot deviate and try another path, because this will make you an easy target for the closest aggressor.
If everyone plays for some other type of victory, then you find it profitable to deviate and conquer everyone else before they reach their culture/science/religion goal.
Sure I can agree with that, anyhow I'm in with @AmtracQuebec that this shrinks alternative goals too much.
Problem in my eyes is the competetive 'only one can win' all MP players are aimed on. If we try to get this a bit more fleshed out with real-life arguments, it would probably lead into other direction.
No one would ever go for decission between winning or elemination. All ppl are just aimed for surviving and having a good life. This is the basic reason for forming tribes, nations, empires aso.
So, if I do not count it wrong, it is needed to have some kind of better balanced end-game results, so not only one could be the winner...or at least not be the only winner.
How about some alternation:
- Let's say the domination victor get's less points the more he extincted other empires.
- Let's say the 2nd and 3rd 'survivor' do not get much less victory points then the actual real winner (according to the line before).
- Let's say count all points together into a Civ6 ladder system. Probably official or semi official...like those winning points I earn in SP...so you could compare on a long run.
- Let's say the other offered winning styles do earn way more victory points than simple domination.
(- There are quite more options thinkable, like accepting minor status instead of fighting till extinction will grant you huge ammount of victory points your domination victor earns at the end. Plus 'rebellion' against your master will grant you extra points aso. Just some thoughts, how such system could show up in the end. My goal in here is to keep once defended players inside a game and give them an opportunity to still earn victory points, no matter how destructive they have gotten conquered. At least a junior-partner might change the balance for it's master on the end game battles, so they still earn victory points for ladder system.)
Then you might get the kind of MP you (and probably me and others) would like to play.
This could be done by external ladder, DLC or mod...