Weapons integration…..why so difficult and costly???

Reading the new Rafale thread got me thinking about weapons integration and why it is apparently so costly and time consuming. One post in that thread stated that the Rafale has mostly French origin weapons integrated onto it (MICA, AASM, Exocet, etc.) and that a stumbling block to export success is the fact that other western origin weapons have not been integrated.

I thought that MIL-STD 1760 was supposed to make weapons and stores integration much easier than MIL-STD 1553 systems did. I understand that weapons seperation testing is critical to ensure that loads can be released without impacting stability and that this can be time consuming. Other then that I was hoping some of you have some insight that can help me understand why this is such a laborious, time consuming and apparently costly process?