Only knowing the relationships between words alone would just be a poor proxy for knowing the meanings of the words, e.g. what real world concepts the words attempt to represent. You might be able to get pretty far with this technique, but I would bet a lot of money you would not be able to get reliable, in-depth human level communication. The system needs to have an understanding of the world.

And then there is the fundamentally dynamic aspect of language, which strengthens the need for a rich understanding of the world that words describe and convey.