Why can't we add WABA number to Instagram account?

While trying to add your WABA number to your Instagram account, you may find this kind of warning

We have raised this issue to Facebook, and unfortunately, the number that has been used for an Enterprise API account, cannot be used for another system once it registered.

Similar things may happen within Facebook and any other apps, it's mostly because WABA cannot interact with other WABA or WABA has special preferences in Facebook Ecosystem.