After looking into aircraft report data at Aviation Weather Center, I realized that PIREP is usually reported inside of US; on the other hand, AIREP is typically reported outside of US. I was wondering if there is any specific/particular reason why AIREP is not available at US territory?
Apples and oranges. A PIREP is a report of current weather conditions at a specific location. An AIREP is primarily a position report, required of some aircraft that are not under radar control. Whereas the primary purpose of a PIREP is to report unusual weather conditions or things like turbulence and are not mandatory, AIREP's are in many cases mandatory and weather information in Section 3 is optional.