Everyone uses the same engine over and over. Starting from scratch instead of iterating on your previous engine is the exception, not the norm.
Sure but I wouldn’t call
making their games as extensible as possible
hard work since it has always been like that.
It absolutely is.
If you don’t make extensibility a core philosophy every step of the way, it disappears very quickly.
Bethesda would need to completely rework their tooling and engine to block out this core philosophy.