I've made this comment too many times, but...

A lot of people don't seem to understand how engines work for game development. I'll preface this by saying that I'm a game developer. Creating a completely new, completely custom engine takes a MASSIVE amount of time and money. So generally speaking, companies either:

1. Use someone else's engine and modify it.
2. Create an engine that is versatile and license it out.

Much more often it's the former option, especially nowadays. It's much easier, faster and cheaper. If a company does choose to create their own engine for themselves only, then they plan to use it for a very long time.

Now here's the important part... It is nearly always a better idea to modify an existing engine than to start over. Engines are written in code, just like any game would be. Which means, you can edit the code. Any feature that you want can be put in. You want Volumetric Lighting? Then put it in. There is no need to create an entire new engine just to add a few new features. Yes, there are exceptions to this. Two reasons why a developer should use a different engine entirely:

1. If you need the engine to be written in an entirely different language.
2. Or if its cost effective to create/license a new engine instead of updating the current code (rare cases).

So what's the problem with Bethesda then? It's not the engine as a whole. Their engine is perfectly fine and creating a new one would not be a smart decision. Instead, they simply need to add in some new features and upgrade current outdated ones. They actually did a lot of this with Fallout 4. They added TONS of new features for graphics and physics that weren't in the engine for Skyrim. A few extra notes to keep in mind:

1. Yes, animations are still pretty bad. This is not an engine problem. This is either a problem of bad animators or their animation tools need updating. Or both.

2. Some of the textures are low resolution or murky. This is not an engine problem. To fix it they'd just have to use higher resolutions for their textures. In all likeliness though, they did this on purpose. Their games are massive and sacrificing resolution in less viewed spots improves performance.

3. Performance isn't the best it can be. This isn't an engine problem. It's an optimization and debugging issue. Generally with updates this improves dramatically.