You're not aware of basic facts now.
1. AIM-120 has a very well known weakness in short range launches. To be specific, it's utterly terrible at them, to the point where there is a clear gap between AIM-9 and AIM-120 where neither missile is effective. This gaps wasn't considered a major issue until Luftwaffe got their hands on MiG-29s and their R-73s and R-77s (Russian equivalents of AIM-9 and AIM-120 respectively) and basically pissed their pants at just how terrible their missiles were in comparison. Brits ended up rushing their ASRAAM project to fill the gap and US has been on/off wavering between buying those and upgrading AIM-9 to fill the gap. AIM-120 appears to be basically unfixable at short ranges for some reason as it hasn't even been suggested as the missile to upgrade to fix the gap.
If I remember correctly the problem was claimed to be either rocket motor that doesn't have sufficient thrust to weight ratio to give missile necessary air speed and maneuverability for window immediately after launch (and has the same problem in the end of the long range launches in terminal phase maneuvering), or the control fins bleeding far too much speed at high angles of attack needed to maneuvre the missile or both factors working together. Both of these are however conjectures rather then facts, as both thrust and turn rates should still be classified as far as I know. It is however known that AMRAAM has not performed terribly well in launches that don't fit its stated "medium range" window, i.e. close range or long range launches.
2. Easiest defence against any radar guided missile is always to run, however that also means that you cannot do anything in the theatre. That is why long range radar launches are essentially a game of chicken - who will turn around and run first, abandoning his launches missile(s) to be useless.
3. Finally AIM-120 has no anti-radiation tracker to lock onto enemy radar. It has a small terminal stage close range radar sitting in the nose cone on a servo, that has to be cheap, expendable and be able to run off battery power (and by extension, pretty crappy).