I don't know enough biology to be sure, but I'm pretty sure that some of our ribs are fairly flexible in this way. I would guess, if we had to come up with a re

I don't know enough biology to be sure, but I'm pretty sure that some of our ribs are fairly flexible in this way. I would guess, if we had to come up with a reason why those bones are flexible and our arms and legs, say, aren't, it's that the tradeoff in ability to exert large forces in a predictable way and taking bigger impacts before breaking isn't worth it. Not that evolution is able to actively design things in that way, of course. More like it was an evolutionary advantage for those ribs to be flexible, but any flexibility in arm and leg bones and other major bones was not an advantage.