That's what I was thinking. But then I was wondering: if it was that obvoius, would there be such research about it?

Some of the research, including this paper, is trying to get at the question of whether a species' sensitivity to the bouba-kiki effect might be at the root of language or not. Since it seems accepted that chickens do not have language in any meaningful sense of that term, finding that they still show this effect decouples it from "the origins of language".

It shows that bouba-kiki is not sufficient, but it doesn't show that it is not necessary.

bouba-kiki has been previously shown not to exist for some set of other primates. given the general sense that they are closer to some form of language than baby chickens, its presence in the latter and absence in the former would suggested that it is not necessary.

all this research could be deeply flawed, however.

Yes, see my other comment about primates https://news.ycombinator.com/item?id=47135674

You do need to research "obvious" things every once in a while. They have this annoying tendency of being proven wrong occasionally.

It's a hypothesis. How would you prove or disprove that it's because of that? (and I would say, a priori, it's not utterly obvious that the brain would relate spacial and temporal frequencies like this)