Because I believe the knowledge graph of Freebase is way heavier than what's needed, and unless you know exactly the type of what you're looking for, it would make a software like Akiva way more complicated to build, imho. (or I missed something)
Since ontology is a very very hard problem, and collecting a lot of facts even more so, I think you should look more carefully into using a structured DB like Freebase that has already figured it out.