It would actually make product development a lot harder. You have three end-user apps developed by different teams, supporting different content, wrapped in different structures. Managing the differences when sending messages from one to another is probably going to result in very weird and inconsistent user experiences, especially in the shorter term.
Also, because of E2E encryption, the server infrastructure really cannot do the sort of content translation necessary to make things seamless.
Facebook already has all the data it needs. What this change would do, is make an average person more aware of this.
It would also help to increase the protection of the single "Facebook Account" to those, who will decide that they still want to have it.
There were many questions such as "Did you quit Facebook in 2018?" in the media. Yet, the question should have really been "Did you quit Facebook, Instagram, and WhatsApp in 2018?".
Essentially, one could share a single table of "users" between Facebook, Instagram, and WhatsApp.
Because the accounts can be easily identified and connected by email addresses and phone numbers, which don't change that often.
This way, a "Facebook Account" could become like a "Google Account", but for social media.
This is exactly how Google integrated YouTube.
It's bad for privacy, but good for everything else.