I decided to do an internet search, and here's what I found out, which is kind of what I was thinking...
In Arizona, air conditioner units are mostly seen on the rooftop. As it is a hot place, the air conditioner units require optimal circulation as well as ventilation.
Furthermore, without needing to go upstairs, the air might be pumped immediately downwards towards the homes and offices. As a result, air conditioners can deliver a better result and efficiently keep the house cool.
https://roofinghawk.com/why-are-ac-units-on-the-roof-in-arizona/