How can a single player campaign cost so much?
Easy, because there's a LOT of unique assets that go into a campaign so that it doesn't feel horrendously boring and samey throughout the 4-15 hour experience that are unlikely to be re-used in any multiplayer mode tacked on.
Lets use an example so we have a good reference point like, say, The Last of Us.
In that shot we can see Joel, several big buildings, various models of cars a few different types of foliage, the road, the wall and fence, some debris.
So what exactly goes into making each of those?
Well, for character models, they're usually sculpted in a modelling program in super high resolution:
That's a timelapse of someone modelling a high resolution model of the Juggernaut. The upperbody. Even at a accelerated rate for the video, just the upper body is an hour of watching be created(It took the dude a month of free time). So figuring design changes and various other things, we can figure a professional could take a few days to hammer out a high resolution model for a single character.
So what happens next? Well, then they need to do ANOTHER model that's a lower resolution so that the game can actually run at an acceptable speed. It's lower resolution, but now we've created two models, both of which take time.
After that, the low poly model is 'unwrapped', in that it's 3d topography is split down until it cleanly lays on a flat 2d surface for texturing purposes. Once that's done, they'll "bake" the high resolution model onto the low resolution model to get what's called the normal map. This lets the low poly game model have the same detail level as the high resolution mesh without the ludicrous polycount.
From there, a texture artist will draw the color texture, and then a specularity texture will be made, which defines what parts of the model are shiney and how shiney.
Pretty much every single 3d asset you see in the above scene went through at least this much work. Every bush, every building, each chunk of debris. Every car. The wall. The lightpost.
Characters, however, also need to be rigged for animation. Someone has to add in a skeleton to the model, and tell each vertex in the game model what bones it uses for animation. Then you have to record animations for the characters. The more fluid the character animation, the greater number of animation clips that need to be recorded. Games like assassin's creed have several hundred and I believe Max Payne 3 had nearly a thousand so that they could blend between them on the fly to get that smooth motion.
Each character animation clip in a AAA studio is motion captured, using an actor performing the motion, often several takes. This then has to be cleaned up and processed by an animator for actual use in the game.
Stuff like foliage also requires additional processing so it can do stuff like sway in the wind.
So now think about when you played through the story of The Last of Us.
How many buildings were there? How many different cars. How many different trees and bushes? Unique signs and billboards? Highway models, blockades. Building interiors with furniture. How many unique character models were there? How many different fungus zombie models? How many unique bandit models were there?
Each and every one of those went through most all the steps listed above. Also consider that an artist needs to create the smoke particle effects, clouds in the skybox, a sound engineer needs to create and process gunshots, footsteps, impacts, dialog.
Then for the cutscenes, they hired actors to levy a powerful emotional performance. They did a full acting routine comparable to some movies for all the cutscenes, and then animators had to clean up and process all those animations too, and sound guys to clean and process the actor's dialog.
Then there's the supporting crews like the guys that go out to record sounds, take pictures of surfaces to help with texture creation, guys working the motion capture equipment and directing the actors.
Sure, character models will get re-used, as will weapons and basic sound effects. A good number of assets too, but all the work for cutscenes, unique characters based in the story, dialog that is only played during the campaign, unique enemies, locales that AREN'T re-used in the multiplayer maps, etc. All of that is sunk cost just for the campaign.
It's astoundingly easy to imagine how they burn through all that cash when it takes that much work to create a single animated model, let alone the hundreds or thousands of unique assets you see across the course of the game, and thinking of the hundreds of people that dedicate tens of thousands of man-hours to produce those assets, all of which takes time.
If a developer did it right, you'll never really think about all this because it just feels 'real', but if you really stop to consider all the work that goes into making even the most basic AAA level that you blow through in 20 minutes, it's pretty surreal.
