Why is that air conditioners must be placed on windows?

Question

Why is that air conditioners must be placed on windows?

Answer 1

Air conditioners work by extracting heat from the air by compressing the air. That heat needs to go somewhere (the first law of thermodynamics). Having the AC by a window is simply to allow the heat to escape to the outside of the building.

Answer 2

They don't - you could also cut a hole in the wall. However, air conditioners need to have one component in the room to be cooled, and another in the space into which heat is exhausted. Thus, the typical "window unit" air conditioner, which cools the room and exhausts the heat to the outside.