The point is so many open source alternatives already exist to Facebook and WhatsApp, all of which can be locally hosted and can federate where required. Local rules can also be set and managed for these networks, and they can be easily branded to suite Ethiopia. Open source means you can see how they work, and even change that if necessary.
Federation means you can host and manage your own instance, and decide whether to open and federate it across other social networks and instances. You have full control over it, and it should not be necessary to create yet another social network from the ground up.
See Ethiopia to build local rival to Facebook and WhatsApp
Ethiopia has begun developing its own social media platform to rival Facebook, Twitter and WhatsApp, though it does not plan to block the global services.