Why cars rust

Salted roads
leaky windshield gaskets
soggy wet carpets
salty wet shoes
parking on grass or stone
vinyl tops that wick up moisture with only primer underneath them
stone chips with bare metal exposed
frame rails that have holes that easily allow dirt and moisture in
door drainge holes that get clogged
cowl drainage holes that get clogged
anyplace two pieces of metal are spot welded together is a potential place for rust to form
but what it all boils down to is anything made from steel that is exposed to oxygen and water will eventually rust