Palem I spent a while looking for this post to reply to it because I've been thinking about it since I first read it.
Some reasons:
- There's a clear winner for wars, whereas hostiles may have no firm resolution one way or the other. Wars end when one side gets totally beaten or decides they're done.
- Wars demand more activity than a hostile.
I also think it counts more because it radically changes the game dynamics. If it didn't mean more, TFC wouldn't have to worry about pushing the button, but you do; I assume primarily because of the attack times. Hostile changes these mechanics slightly but not as radically as war. Prop and AW opening up is also a significant change to strategy.
It's just a bigger achievement than milking a hostile, even if you are successful at making their lives miserable.
Finally, I think your own recent post in the Masks thread kind of answers your own question:
You're right, it's a huge game-changer for a top kingdom to lose a war. But it might be manageable if they just go hostile. It's just clear that the effect on the game a war has is greater than a hostile. Which is why it's worth more :P