I find that I often guess very wrong on the early congress resolutions, so I don't get that base of points. And then since I don't play apocalypse mode, there's usually only a couple aid requests in the game, so I hardly ever even come close to a DV. I've won once by it (as Georgia), and that was the only game I truly tried to manage the votes, made sure to always vote how I felt the rest of the world would vote, and had so much diplo favour that I could push through what I wanted at the later resolutions.
I do think the AI is too easy to guess on those diplo votes (as mentioned in OP, stuff like city centre production they usually just put one vote each into, and many others they do as well, so if there are 5 AI I know that I just need to put 6 votes in myself and I have the pick of the litter).
I do think there are other things they could do. Like, maybe all the emergencies should basically give you a choice if you win - you either get 1 diplo point, or you get Reward X (ie. +1 gold per envoy, free great people, etc...). In that way, basically you have to give up something if you want to go towards a diplo victory, but if you're not, then you still get a reward. Personally, culture victory is the one I tend to slip into easiest. Mostly I think that's because when I get late in a game I'm winning, I just start wonder-spamming, or buying great people, or digging up artifacts, and combined with the monopoly bonuses, that can really put you close to the culture win. More than once, I get to within 10-20 turns of a CV and could easily have won by that if I slotted in some of those policy cards or reorganized a few things around my cities, in order to win the way I originally intended to.