I would have guessed that it is just due to humans being very visual beings.

Well, we're also verbal beings, in fact most of our explicit rationality ability is verbal rather than visual.

I'd be really interested in seeing a way of testing your hypothesis.

I haven't thought of a good way to do this. However, one way to test your "it is just due to humans being very visual beings" is to take advantage of the fact that people differ greatly about how visual they are.