On one hand my first guess is: It's just the way it is, because people like new thinks, same as they hate them. Designs and layouts are improving (or changing, if you want to name it more neutreal) all the time. Means: A new "design language" comes up, e.g. the current material design. It looks totally different to the current design-language.
Depending on the impact of the one who creates this new design language, eventually it becomes a new standard. Every website (or in general every UI) not implementing this new design language suddenly looks different. After a period of time, and after more and more UI's are implementing this new design language, this difference is simple being perceived as out-dated.
And that's the point when the designer decides to re-build it. And somewhen in the future, you cannot resist, because all those libraries are based on this new design.
On the other hand I assume, sometimes new features just requires new designs - form follows function. A growing page could require a search form or sub-menus and then you need to think about a re-design.
Summarizing: It's just a matter of time. Try to create and distribute one of those old MS-DOS apps, those that you are controlling with the keyboard only. Maybe they would still work in some use cases, but could they provide all the features the user needs, would they run on a brand new Windows 10-machine or even a smartphone?