Their engine is not the issue. Starfield in particular needed a lot more investment into making their engine work for that style of game, but it is functional for FO and TES. Switching to UE would require building a ton of tools, creating a lot of new tech, and having the team learn to use it. UE isn't just some perfect engine you can swap over to. It has tons of issues and things you need to learn. The bones of it are as old as the Creation Engine's too.

My biggest issue with Starfield was the writing. The stories were boring and mostly didn't make sense. They also forced you down one or two paths, even when other options should have been available. This is also true for their other games, but much less noticeable.

They've also been removing the player's ability to make their own fun in the world, and instead you just have to follow what they've given you. For example, no spell crafting in Skyrim, and very limited enchanting. Why? They still aren't balanced. They're just less fun and interesting.