Maybbeyou took and shot yourself in battroid or gerwalk*...
They are modular attachments in the sort they are just mecha-sized hand-held rifles, but I doubt their mounting/holstering(?) points or brackets are universal. So where are you gonna stow another VF's gunpod in fighter mode if it is of a different type? Hmmm?
*This reminds me of a Battletech house rule, if you missed a to-hit roll with an arm-mounted weapon at point-blank range, you had to make gunnery skill with a modifier. If you failed that roll, you shot yourself!