From my understanding, it’s just like a legacy code base. It is just easier/cheaper to start from scratch than converting the entire thing a piece at a time.
That's mainly how renovations go. Just easier to have the demo team demo and then have the trades come in and do their thing. Working around existing infrastructure and integrating with it is unpredictable and messy. Not to mention, once you touch something you have to bring it up to code so best to leave it alone or replace it entirely.
I did a renovation once where the idea was to preserve a few of the old wood windows that would be lost due to the nature of the renovation. The original idea was to reuse them and move them into the new renovated space and install them. Turned out it was going to cost more to carefully remove them, fit them, etc than just buying the highest end Marvin windows. So new windows were purchased and the old ones are used for parts now.
