Why do tin cans rust like steel?
Because the are mostly steel. The “tin” in tin-can comes from the tin/lead solder used to make the can. An actual tin-can would be a tad expensive. I think I remember seeing someplace (great cite, huh) that they would cost $5-6 dollars each.
Because most of what we commonly call “tin” cans ARE steel, actual tin having been displaced as primary canning material way back in the early 20th century.
Also, stuff like ‘cookie tins’ (you know, those boxes that special-edition baked goods, or those Danish Butter Cookies come in) are made of ‘tinplate’, which is a thin sheet of steel with tin plated onto both sides (or sometines just one). If you sratch the surface of tinplate with, say, a nail, the scratch and the area right next to it is open to rusting.
They aren’t, and I believe never were, cans made of tin. They were called “tinned cans,” meaning steel coated with tin, shortened over time to “tin cans.”
http://www.solidwaste.org/recdetin.htm
http://www.uoregon.edu/~recycle/after_collection.html#tincans
I think most cans with a seam are welded today. While tin is a component of solder that is not the answer to the OP, cans were originally made of tin and then tin-plated steel.