With the tremendous amount of time, energy, brain-power and money that has been invested in 3D engine middleware development in the past 10 years, it's really absurd a major game studio still manages to release a game that stumbles because of technical issues. It's not as if 3D engines and its subsystems are new and development of these is uncharted territory. It's also not the case there's little to no information regarding how to architect a proper 3D engine and its subsystems so they work together nicely.
It all reeks of egocentric douchebaggery inside the studio, like we also see in other Bethesda studios like the one making certain fantasy / post-apocalyptic RPG games, where ego-driven choices dictate what to do and what to use instead of reason. If you use an engine that's derived from a version of which another derivative also was derived but has major advantages, if you can't move your assets, scriptcode etc. to that other derivative, you clearly didn't understand what the whole point is of 'middleware', namely you pick something off the shelve and outsource the heavy lifting to that middleware. Apparently if such a move is technically impossible you changed fundamentally so many things, one seriously has to question motivation for that as it's a waste of time and money.
This is something that EA with Dice's Frostbyte has understood very well: they invested a lot in the engine and it's now so adaptable that studios can use it for practically anything: what they need specifically for their game they can build on top of the engine and leave the engine as-is, minor changes aside. Bethesda is just too lazy in this, they still think if you make a game you apparently build the engine to run the game on first, or heavily modify an existing outdated engine because apparently the game is so unique no-one ever wrote the required technology.
Most people aren't programmers. They don't realize any of this. They already find it magical that you start a game from the steam client and through some magic, you can move a person around in a 3D world. To use an analogy: Bethesda and their studios try to write a new web-browser every time they create a new website, or polish up a massively outdated version of an old web browser for the new website they're creating. that's not how the web works of course, and that's also not how development works anymore. Some wheels are re-invented enough times, and Bethesda needs to start realizing this.
Ubisoft more and more does, and if they can, Bethesda can too. Yes I know it's made by a studio not named Bethesda, but Bethesda feeds the bill. They're the publisher. They call the shots.