Poor Quality Chrome
Chrome is copper...nickle then chrome.
Yes it's the steel that rusts but only after the cheap chrome plating breaks down. Don't blame the steel...unless, maybe, if it's Chi-Com.
We use to lay on two or three coats of copper with a polish job in between coats... that's very expensive to do. Lots of man hours. You don't get that with production chrome.
We had a joint called Murray Metal Plating just a mile from my high school. If you were lucky you got a job there at 14 years old and worked every night after school. One of the benefits was you were allowed to chrome anything that came off of YOUR car... not your buddies.
My buddy had a '54 Plymouth he bought for $15.00. He had rechromed bumpers, chrome dash, inner fenders, grille, all the bolt ons, chrome hood hinges... almost the whole car was primer and chrome.