Many buildings would be extremely expensive to convert into usable space for living. Rooms without windows cannot legally be bedrooms, bathrooms might exist but the plumbing may not be usable for a kitchen or shower, electrical panels might not be wired to accommodate household use, heating and cooling might be significantly more expensive.
A more efficient use might be to convert existing commercial real estate into a multipurpose use (grocery / entertainment / medical / etc) and then build apartments surrounding it.