To make up some numbers: Assume that a paint job lasts 20 years. So if it takes a team of five 20 years to paint the bridge by the time they've reached the end the beginning will need a new coat.

Otherwise every 20 years they would need to hire, train, then fire 200 painters for a 6 month-long paint job project. 200 painters, managers, and etc. who will invariably be less experienced with the finer points of painting said bridge since they don't do it every day.

So it doesn’t rust