I do not know for Spain (and I know that in France it is not as strict either), but in Belgium you have to pass the car to a check every year and if it does not pass it cannot be used on road anymore until problems are fixed.
In early 80s my parents had a 5-year-old Renault 5 which failed that check because of rust So here you were never seeing rusty cars (and they do not rust anymore, or at least not as quickly as before).