every time ai wins because sciense victory is the fastest way to win,
Diplo victory is MUCH easier to win than science victory, just the AI isn't always smart enough / cheap enough to go for a diplo victory when they have all the extra gold.
If you're having that much trouble with the AI, I'd recommend improving your strategy and playing at an easier skill level instead of just complaining here that the victory condition sucks. It's not the game's fault, it's you doing something wrong if the AI keeps beating you the same way, so learn how to play better.
Finally, this belongs in the regular Civ V category, not Gods & Kings, since it has nothing to do with the expansion pack.