Steel does rust, but good metallurgy and coatings help. I live in the upper Midwest where road salt is used quite heavily, yet the prevalence of super rusty cars has really diminished.

Steel items that are not exposed to salt either rust much more slowly, or develop a patina that slows further rusting. That's not quite so easy in coastal areas.

Modern stuff is treated. Old stuff like a roof I had was just thicker than modern. The roof was standing seams at probably 30-40 years old. While there was rust, it was superficial. Takes a long time rust clear through even in Florida.

It's galvanised. These days you can also get pre-finished corrugated iron (Colorbond), which also has a layer of polyester paint baked on.

In my area, corrugated steel is sold with 'zinc-alum' and powder-coating pre-applied.

I think they also apply paint to cut edges and drilled holes during installation.

Its extremely popular, probably seen on around 50% of roofs, and is also used for exterior cladding on a lot of new houses.

(although as cladding, it's usually used for only half the house, alternating with some other cladding)

Despite the name, it's usually steel, not iron. And a newer galvanization process that's a mixture of aluminum and zinc.

the rust looks cool tho: https://instagram.com/totanmania_0