Over the years of its existence, Bethesda has been criticized by many for using the outdated Creation Engine. However, despite its bugs and inability to work with open worlds, the creators of TES 5: Skyrim, Fallout 4, and Starfield are in no hurry to switch to the same Unreal Engine — and here's why.
Gamebryo is no longer a business, it hasn't been for a while. But that engine has been constantly tweaked, updated and refined to do exactly the kinds of games that Bethesda makes: The Elder Scrolls, the Fallouts, Starfield. It's perfectly tuned to that kind of game. The cost of moving into Unreal would be all the upfront development you'd have to make to try and shoehorn what you're already used to being able to do into Unreal.
Another reason was the engine's friendliness to modding. Bruce Nesmith confirmed the obvious guess that Bethesda does not want to lose the modding community, thanks to which its games have practically gained immortality.
They've put in a tremendous amount of effort, the modders, into understanding the Creation Engine and understanding the Creation Tool Kit. If there is a cut over, all that is thrown out the window and it all has to start all over again.