Ok, I watched 3 minutes in and stopped ... so basically the reason is "Oh no ... my profits!"
Seems to completely miss the point ... which is that this is about reshaping the city ... a necessarily bigger task.
Also, I've said it many times ... but this whole office -> residential thing just highlights the robustness issue that the pandemic highlighted. That all of these office buildings are optimised for fitting in as many desk workers as possible and not anything else, to the point that they are problematic and illegal for someone to just live in them, is a huge problem. That the conversion would cost so much money is a design problem, surely, a mistake that someone has to now pay for (sorry landlords!).
The lesson being that there's probably a decent middle ground ... a "generic humane building design" that can work decently well as a residential or working or mixed property ... something that isn't as "efficient" but instead amenable to the flexible needs of a variety of people doing a variety of things.