But.. it’s not the same engine. Not entirely.
It goes like this; from Morrowind to Oblivion, there was a buncha stuff added. During that era, Oblivion, Fallout 3 and New Vegas did indeed share the same engine, with only minor improvements for each subsequent one.
Once Skyrim hit, there was a major restructuring of various parts of the engine, particularly the rendering and scripting systems, which took into consideration all the mods that fans had made for the prior games. If a script extender added a feature in one game, expect the subsequent one to have it as standard. Fallout 4 has about as much of a jump up in that department as the Dovakiin’s adventure had over the Courier’s.
I mean, haven’t you noticed that Fo4 does things which the previous games were incapable of, or required a massive amount of trickery? Like, say, elevators and other lifts? Go check out how they managed to trick things in Mothership Zeta and Lonesome Road.. they’re actually simply enabling and disabling stuff which is stacked over and over upon itself, out of the player’s sight, simulating movement, or moving only a handful of objects instead of actually progressing through a physical space. In actuality, you’re often in an unmoving room the entire time. And this required you to be indoors to pull off, as well. It’s why you can’t noclip into those other “rooms” once these sequences end.. they don’t exist anymore!
In Fallout 4, there’s tons of places where you can catch a ride up to the roof of a Gunner hideout, or make better use of the 3D enviornment than previously allowed. The increased vertical use isn’t just a design choice, but an actual thing the engine lets you do which previously wasn’t much of an option.
I think a lot of the confusion is the way the console commands and data structures are preserved across each subsequent game; but that isn’t new for games or computer applications in general. Hell, remember every time Microsoft Word or WordPerfect changed a document format and everyone flipped the fuck out? There’s reasons to keep these things as similar as possible even when you advance your code in other ways beyond laziness.
Now, that’s not to say they aren’t re-using plenty of assets and programming, they are. But you don’t reinvent the wheel with every single new car, so why would you expect that of a game developer, especially one whom makes game worlds so massive, interactive, and with very particular demands (and a fandom that is even more demanding)? The Witcher 3 is often brought up by comparison, but you do realize it “cheats” more than Bethesda’s games do, right? Try saving, quitting and loading and notice.. not everything is the same as it was. Only some things are actually preserved. This happens during gameplay as well, but it’s even more subtle. While something like Grand Theft Auto (any of the 3D ones), actually is so aggressive at culling things, that NPCs, cars and whole areas can be purged from existence the moment they’re out of sight. Even in some cases when you just turn the freakin’ camera!
Meanwhile, in these games, everything is tracked. Many, many, many things continue to occur when you aren’t looking, or aren’t anywhere even near that part of the world. Part of why cell resets are more frequent in Fallout 4 compared to Skyrim or prior Fallouts is not just to make it more Monty Haul; it’s also to alleviate that tracking. Almost everything has persistence of some sort, it’s the degree of it which is key in improving performance when modding or developing the official content.
In a rather brilliant move, even the save game system reflects this. It’s more like a save state, and is only a barely modified version of the core files. When you save, you’re basically saving a “mod” of the world, whose changes are then applied over the original. This is why such files continue to get progressively larger and take longer to load, whereas in some other titles, the savegames have a limit or even a fixed size. But it does mean everything is where it should be, barring glitches & bugs, of course.
These are areas in which idTech, Unity and even the Unreal Engine lack compared to the current Creation Engine. So while all of those have their advantages, with the first and last having a ton in the graphics department, they also just aren’t as good at making the kinds of games that Bethesda and co. create. With customization, sure, you could probably implement those same features in most/all of those engines. But.. why would you do that, when you’ve already got it done?
tl;dr: It isn’t the same damn engine, though some parts are, in the same way an Intel Core processor from this year can execute the same instructions as an Intel 8088 chip from 1979. And switching engines would require a lot of work to bring it into parity with what the current one can do, as well as years for all the developers to learn it inside-and-out. So unless you want them to take three to five times longer to make their next game, as well as somehow remain in business.. this isn’t going to change unless said next game’s demands also change significantly with it.
Citations, links, corrections and quotes to be added later. This whole thing already took an hour plus to write as-is!
Thank god someone had the technical know-how to say it all- game engines are really complex things often tailored to the kind of games they need to run (You can’t get a scale of a game like ARMA on Battlefield’s Frostbite engine- you just can’t).