Why couldnt Iron Man (from the movie) actually be made from Iron?

Question

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Answer 1

He would be dead.

Answer 2

Pure iron is a relatively soft material. The presence of alloying elements, most notably a small proportion of carbon, is what turns it into steel, with then very interesting mechanical properties.

Beyond that, there are materials that perform better still than steel (lighter with the same strength) but with a cost impact. Given the money Tony Stark has, cost is seemingly not an obstacle, so he can go for the best material.