They are reworking their tooling and engine constantly.
If they weren't making a deliberate point of making extensibility a priority, it would disappear on its own as development that didn't make it a focus left it behind. It doesn't just magically happen. It's because of good process.
It's the core because they spend a sizable portion of their resources on making it that way. Every line of code that doesn't explicitly keep interoperability in mind is a line of code with the potential to catastrophically break it.
It's not something you can do, then you have it. It's like exercise. The day you stop it starts to fall away.
I mean do you think they’re using the exact same tools that made daggerfall and morrowind? It’s iterated on and built out every game. The current toolset looks very different from what they used even for Skyrim.