The battles are limited to 25 turns so your mounted/hasted units can run away from slow OP enemies like orc warlords.
The way you say this makes it seem like you think it's a good thing. It completely broke multiplayer (to say nothing of singleplayer) and more or less single-handedly resulted in flyers landing at the end of their combat turns in AoW 3.
I also don't think the AoW series is a particularly helpful comparison here. I mean, don't get me wrong. I generally prefer AoW to Paradox or Firaxis games. However, there's a key difference: AoW is on a much smaller scale. AoW revolves around one thing and one thing only, and that's combat. There's empire-building, there's magic, there's economy, but all those things only exist to help you win battles. The AoW series has great tactical combat because it's literally the focus of the franchise.
This is not the case in games like Civ or Paradox games, which have a much broader scope. In fact, arguably it's reversed - where in AoW empire building was the tool that led to more combat, in these games combat is the tool that leads to more empire-building. Combat is the means to the end, rather than the end itself. I don't think you could successfully add tactical combat to a Civ or Paradox game for the same reason that I don't think you could successfully add century-spanning empire building to an AoW game - you're diluting the things that make the game great, in hopes of making the mediocre part of the game slightly less mediocre. Civ V (yes, even with expansions) is, I think, a good example of this.
I don't remember the EU/CK/Civ games taking into account seasons. Winter attrition should be a bitch, especially for doomstacks.
I wanted to dig this comment up. As others have mentioned, it's false, and there is in fact a winter penalty in EU and CK. Also as others have mentioned, it's not a huge deal. You'll take increased attrition, but your enemy will as well, and later in the game it will hardly be more than a drop in the bucket. The reason I bring this up, however, is because I think it's a question of scope. Seasons can be hard to effectively simulate when the game spans centuries and each season can often only last for a few minutes. They can also be difficult to simulate when you're playing a world-spanning game where the same month will have drastically different effects in different parts of the world.
I think more than anything, though, it's a fundamental design decision. I think that heavily penalizing winters could very easily make the game into too much of a Red Rover simulator. Already it has elements of that due to the massive bonuses terrain can confer to the defender prior to generals with decent maneuver (in EU4), or forts (in Vicky 2). I think the reality is that strategy games need to generally avoid penalizing offense too much. Turtling should be sub-optimal, because a game that rewards turtling is a game that often takes three times as long as it needs to. This can lead to the game being pretty ahistorical, but it also makes for a more interesting and exciting game. I think this is relevant to the thread at large, because I would argue that doomstacks tend to favour the attacker. They allow the invading nation more control over the location and terms of the war without having to worry about things like supply lines getting cut off, or guerrilla attacks.
One other thing I'll say in defense of the doomstack is that they are fast and they are neat. I don't think I've ever had an experience in a strategy game more tedious or annoying than trying to move a large force in Civ V. Even managing multiple smaller stacks in games like AoW or Warlords can get bothersome at times. The doomstack allows you to move armies quickly and conveniently. Since wars often come down to doomstack vs doomstack, it also allows most conflicts to be resolved fairly quickly. One massively unfortunate aspect of non-doomstack combat is that it will often reach a point where one side will very clearly win, but it's still going to take quite a while for them to achieve that victory and mop up all the nonsense lying about. Doomstacks help minimize this. Wars become (roughly) a process of "Two armies enter, one army leaves." Like I said: Neat. Tidy. Is it strategically shallow? Often, yes. Does it feel a bit unsatisfying? For sure. But for games where combat isn't necessarily the main focus, it helps make combat into something that can be resolved quickly and relatively painlessly, and I think, overall, that's worth the occasional bit of shallowness.