Possibly related: Meta is removing cross-platform chats between Instagram and Messenger [1].
I wonder what changed. Maybe testing showed that people actually prefer them to be separate?
Let's consider you have Instagram users on one-side, and Facebook users on one-side.
As long as you have at least one contact using only Facebook (like your parents), then you have to be active on both platforms in order to talk to your contacts.
If the two platforms would be unified, then you would be active only on Instagram for example.