How difficult is it for nature to create rectangular shapes? I don’t actually know how to answer that question.
It is interesting to ponder it, though. Look at yourself. How many actual (non-mental) edges do you posess? How many corners or other pointy bits?
No beak, no claw (unless you wish to have one and put the lack of effort into it to create one), just teeth.
I probably missed something. Truth is, those structures aren’t really rectangular either. They are intriguing to say the least and I highly recommend you to click through to the source to read.