Objects that have sharp edges generate higher frequency harmonics when agitated, because lower-size features resonate on higher frequencies (like shorter strings ring on higher pitch). Objects that are round resonate on low frequencies only. The "kiki" sound has more high frequency content than the "bouba" sound, and it's no mystery why the brain associates one with the other.

In nature there's few things laying around that resonate particularly well.

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