It absolutely is the fault of the engine, but that’s not because Bethesda is incompetent or anything. It’s actually a pretty complicated issue, but yes, it is due to the engine. I wrote a whole explanation for someone else who was parroting the “it’s not the engine’s fault” bs that Pete Hines & Todd Howard started perpetuating a few years ago, so I’ll put it here for you and anyone else:

The problem isn’t the engine itself, it’s that Bethesda hasn’t given it the attention it needs.

Unreal Engine 5, for example, is built from the original Unreal Engine. But there has been so much work put into it that it’s nearly impossible to tell. Meanwhile, the creation engine literally has some of the same issues that the Gambryo engine had back during Morrowind.

To Bethesda’s credit, this isn’t entirely their fault. There’s a reason that proprietary engines have been dying out in favor of engines like Unreal, and that’s because maintaining and improving game engines is incredibly time consuming and expensive. And unless you’re directly profiting off of your engine, like Epic does, you don’t have a massive incentive to endlessly polish it. Doing so is time you could be spending working on your next game, which you do directly profit off of.

Personally, I want Bethesda to keep using the Creation Engine, or whatever they turn it into next, because of its incredible mod support. However, it’s nowhere near as polished or advanced as other engines, and understandably probably never will be. There’s really no easy solution imo.