Players like Acken completely outclass the AI, that's why...
And I fail to see your logic (unless you assume everyone can play like Ironfighter or Acken but they just decide to play badly so they barely win... or lose); some players are truly completely outclassed by the deity level difficulty, including myself on some starts.
You DO have to agree though that sometimes you'll find deity Alex spending all his gold on CS, winning DV instantly once you enter info, and some other times he won't bother as much... That's what's weird about DV the most; it's almost a dice roll depending on whether or not the AI decides to spend their mountains of gold on CS.
Now I have no intention to pick a fight with anyone, so chill out. I'm hardly the only one here who thinks the same way about DV. Another poster once said a 10yr old could win DV on deity.
As for the 2nd part of that speech you just made there, very well, I accept your challenge
(how hard can an X-com rush on every cap with spies planted beforehand be?); while I can't pull off a CB or XB rush because frankly I've never done it on deity, artillery rushes should be easy enough. The main reason I don't "do" war mainly is because the unit cycle takes forever on my poor overheated laptop (and besides by my personal taste, empires that are conquered through war are, well, quite ugly... small cities which won't grow and a bunch of fallout and pillaged tiles).
Now if you think you can win peaceful CV on deity by my rules, and it's easy (realistically though, if you think it's really easy, show me a map where your CV is quicker than your SV or DV), OK go ahead; I'll say it straight that it's not that hard if you know what to do (but I think you'll find you're going to have to change your playstyle by a lot) but I DO think it's harder than the SVs and DVs on the vast majority of maps. (I can think of certain deity maps where SV is SO much easier than CV... I can't think of one in which the opposite would be true)