One problem is that the owner of the land, the owner of the building, and the developer are generally separate and distinct. In my experience the vast majority of property owners - especially those with the wherewithal to own prime CBD real estate - tend to be quite happy to sit on unproductive land for a very long time. If any of these buildings get razed, my guess is that many landowners would rather turn it into a parking lot for half a decade than let a developer turn it into something cookie cutter.