Yeah used the Apex engine, probably that's what they are used to using. Or ID Tech is not suited to open worlds perhaps?
id Tech 5/6 sure wasn't (mostly because of Megatexture would saturate the bandwitdth available for streaming other things, but also map size limitations). id Tech 7 is quite a bit more suitable as they got rid of megatexture improved map size and improved streaming a lot.
It's certainly not the best engine in the market for it mind you, but software necessities can drive engine development to find solutions, so idtech would improve on that front if it had to.
Speaking of software necessities enforcing changes on an engine, I have no doubt bruteforcing Fallout 76 through their current engine, as crazy and narrowminded as it was judging from the state the engine was - certainly made the engine internally very different and more up to date than the previous FO4 revision (specially in the years of fixes post release). That doesn't mean all it's quirks and convolutions are gone though specially seeing what happened was a form of survival mode, not something like a proper engine development process with a proper roadmap.
There's no excuse for the graphics pipeline not to be up to par if revamped enough (IW call of duty engine roots are Quake 3/idtech 3 for instance), just like they would surely be able to pull their "tons of movable objects" elsewhere with more total resources available/better multicore support than their own offering. Saying otherwise is acting like their engine is very efficient which it isn't, the thing it is is specialized, and there's difference.
Still familiarity lowers dev costs at least initially/theoretically, as it's easier to prototype and get the ball rolling; this is the reason Valve did HL:Alyx on the aging Source Engine just last year, and there's plenty other examples.
Main reason they don't want to move from their engine is down to their ancient non-efficient
Papyrus scripting language.
And yeah, as some pointed out Red Engine has similar behaviour is a lot of things yes. Graphics pipeline is way more polished but the way some things behave makes it very obvious that the reliance on scripts is similar. And I pity the devs who have to use it for it, gives you more "liberty" to implement whatever you want without requiring specific engine development to pull, yes, but also makes everything manual/case-by-case scenarios. Things become very complex (and inefficient) that way.