You may have noticed that most air-conditioning units (wall units, cassettes, ducted units) are always placed at the top of rooms. To explain this, we will need to understand a little about the concept of heat transfer and Physics.
There are three main methods of heat transfer, mainly: Conduction, Convection and Radiation. The main process of heat transfer in terms of an air-conditioning unit is Convection, which has to do with moving fluids like air.
As air is cooled, it contracts, become denser and sink. Warmer air at the bottom is displaced and due to it being less dense, it rises. This process continues and sets up a convection current that will allow thermal transfer to occur.
Since cold air is emitted from the A/C unit blowers, the unit has to be placed at the top of a room to allow a convection current to form. This placement provides an efficient method of cooling the room. If the unit were to be placed near the floor, the cool air would be "stuck" at the bottom and the room would not be cooled effectively.
While there are some A/C units that "throw" the cool air upwards to generate this convection current, the bulk of A/Cs available are situated at the top, allowing Physics to do the work!