Why do you think air is transparent?
Air allows almost all the light that falls on it to pass through.
Any object around us can be seen because light gets reflected from that object and reaches the human eye.
Once light reaches our eyes, signals are sent to our brain and human brain processes the information and the object is made visible.
Air is made up of gases which allow almost all the light that falls on them to pass through them. So no rays are reflected from the air and we are unable to see air. This is the reason why air is transparent.