Generally, it's at a mid-point or higher of the side of the tank to minimize the water accumulated at the bottom. If air was only air then you can place it anywhere but you got moisture/water to worry about.
I wonder if someone can photoshop a nice rendering of a high end compressor and send it off to quote at a few Chinese vendors with the air output at the bottom of the tank. I'd be curious to see if it makes it to market somehow thinking they copied a super high end compressor