You had me up until this point. Sadly, office buildings are constructed quite differently from residential buildings, and it really is hard to convert them over. It makes more sense to leave them as non-residential space, even if vacant for awhile before finally being reoccupied, and then to separately build brand new purpose-specific high rise residential buildings (which will be nicer than a converted office building ever could).
And another problem with the conversion plan is that office buildings aren't fully vacant! Here in NYC, the office vacancy rate is just under 20% (and falling). But that means it's very hard to find a single building that's 100% vacant to convert over. Most buildings are mostly full with long-term commercial tenants on long leases. Yes, some few floors might be vacant, but many are not; you can't exactly kick out the existing tenants that are making the building mostly full in order to convert everything over. It doesn't make sense and it would incredibly costly. Plus, the buildings with the highest vacancy rates are the oldest ones with the worst conditions (i.e. not prime commercial space); these are less attractive for conversion for residential.