I never said there wasn't. However, protocols are made considering plane's and pilot's security. Once a missile's armed and then fired and if the plane is upside down....how'd you release it and expect it to go many meters below due to the gravity before ignition?? If you are upside down, it can't be released like like that as you'll be underneath the missile. It would ignite from it's current position or may be a couple of feet from the jet. Meaning the jet will be hit with the smoke and degree (that could happen easily if the missile is so close).
Under this situation, you are firing like this due to the angle of attack and being in combat....you may not have the time to straighten the aircraft to 'properly fire' the missile.
Jets with weapon's bay (Stealth jets) are a different story. They don't show up on the radar so the chances are that no one is seeing them clearly when they open up their weapons bay and make it rain with AMRAAM or whatever else. Plus, for other planes, during test firing and training, they always fire from a straight normal position. In combat, that position could be anything but straight and simple.