Think of an old brick building with several stories and a window per floor vs a new steel + concrete building with windows spanning multiple floors.
The "older" builder like the converted one in the parent post has small windows, allowing easy subdivisions. Newer buildings have windows spanning multiple floors and need to be retrofitted and on a skyscraper that comes at a huge cost.
The bigger ticket item is the plumbing and ventilation, and to some extent the electrical. Ventilation is needed around the cooking area and washrooms, adding that to a building not purposed for this is challenging (where does the "contaminated" air go out?).
It's often cheaper to bomb down the building and start over than doing a conversion on a new highrise. You'll see this often where they gut the entire structure and floors, keep a few walls/supporting structure, and build new.