Facebook to Integrate Instagram, Messenger and WhatsApp

The important bit here is that, they will remain individual applications, with a unified underlying architecture.

It is a smart move, internally, to leverage a single framework for dealing with similar services. Less training, and greater lateral agility for employees. Developing a new feature for Whatsapp, and need some extra man power? Shift some workers off one of your other messenger services. Right now that may not be possible.

I decline to believe at this point that there is another corporate reason other than that.