no crawl space between walls and floors and ceilings in original design of buildings make it tough to built ac ducts. moreover in highrises with over a dozen units of dwelling the central ac with its limited cooling coils and compessors cannot adequately pump copious amounts of cool air to each and every unit. a distributed cooling coil system (fans and coils per room) with centralized compressors on rooftops will be more optimal. the plumbing between bulky compressors and smaller cooling units is also less conspicuous and can be better hidden in ceilings and walls.