Remember that we humans get "cheats", or at least advantages, at all difficulty levels. The AI are required to roleplay their Civs, even if that's not in their interests; we're not required to do that. The AI can't reload saved games; we can. In prior iterations of Civ, the devs (e.g., Soren Johnson) said they prevented the AI from using annoying "win at all costs" tactics. Maybe Civ 5 is different, but I doubt it. We complain about AI diplomatic tactics, but I suspect there are more constraints on AI diplomatic behavior than on our own.
In any case, I don't mind playing a game against a weaker opponent who gets a handicap. In chess, the stronger player might get a bishop or even a rook. I don't have any philosophical objection to that, so long as the resulting game is a fun challenge. If it's given enough units, the AI gives me a fun game.
And yes, the Steam stats indicate that few of us are winning games at the top difficulty levels. Yes, I know some people don't bother to play out their "obvious wins", but there are what, 50 times as many recorded Warlord wins as Deity wins, and I doubt Warlord players are 50x more likely to play a game to its conclusion than Deity players. Put another way, perhaps Steam is under-reporting Deity wins, but if so, it's also under-reporting Warlord and Chieftain wins. And even if fully 3/4 of all Deity players get bored and don't play out their wins, that still means a miniscule win rate at that difficulty level. The game seems to be presenting plenty of challenge to most players.
Well, I just won a science race on King. It wound up with just me and Catherine with capitals left, and her army was a lot larger then mine. So with one part of the SS left to build, what does she do?
Why, she decides to go attack Greece (which has one city and no capital), and then the Ottomans (which have one city and no capital). So sitting merrily alone with nobody bothering me I build the pieces and win.
The AI being stupid isn't something giving the AI more bonuses can fix. She needed to take my capital to stop me from winning, AND to win herself. Why did she leave me alone for 30 turns instead when she had the millitary to do it (at least in theory, it's not like the AI is good at war either).
Giving the AI increasingly silly bonuses is not a substitute for an AI that tries to fight back effectively.