Why are some missiles incompatible with some aircraft? If fighter A has better radar and missiles than fighter B, then why isn't it possible to simply integrate the better radar and missiles into fighter B?
Several potential reasons.
- The aircraft may lack the electronics or software to operate the weapon.
- The missile may not have the required connectors and other hardware to interface with the aircraft
This prevents many Russian/Soviet style weapons from being used on American aircraft. It also prevents a lot of French weapons from working on aircraft being built anywhere else.
This was a major factor in the Dutch decision to purchase the F-16 instead of the Mirage 2000, as the latter would have meant not just replacing their aircraft but also replacing most their entire inventory of missiles, rockets, bombs, and everything else (and all the maintenance equipment).
Sometimes the differences are small enough that they can be retrofitted, for example if it's only a small software change. Sometimes it requires new hardpoints to be added to the aircraft, or adapters to be added to existing hardpoints. And sometimes the changes are just too great to make them worthwhile. With aircraft that use internal storage (like the F-35) an additional factor is the available space in the weapons bays.