Also from an engineering standpoint, I would think that just metal-izing the skeleton wouldn't be very helpful. You'd just switch to breaking all of the tendons and ligaments and other connective tissue first instead. So you'd need to strengthen those up too, and in a way that is still as durable and repairable.
Then all of this stuff is adding more weight, we're going to need bigger or more powerful muscles to move it around well. Then we'll need bigger/better heart, lungs, circulatory system to fuel that extra muscle power, and improved digestive system to get more energy for that stuff into the body. Skin is seeming kinda fragile with all of this extra bone strength and muscle power, better toughen that up too.
I think I have wondered far more often why bones aren't made from a substance stiff enough to act as anchor points for tendons, but also flexible enough to bend over and then snap back to its original shape without breaking. The bending stiffness and yield strength seem very important for the types of stresses on a biological organism.
So why aren't our bones made out of polyimide-polyamide-fiberglass composite?
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.
Then all of this stuff is adding more weight, we're going to need bigger or more powerful muscles to move it around well. Then we'll need bigger/better heart, lungs, circulatory system to fuel that extra muscle power, and improved digestive system to get more energy for that stuff into the body. Skin is seeming kinda fragile with all of this extra bone strength and muscle power, better toughen that up too.