Why doesn't rust protect iron the way other metals' oxides protect them?

Because rust is flaky and crumbly: instead of clinging to the metal as a tight seal, it falls apart and leaves the iron underneath unprotected. Aluminum, copper and tin are luckier, because each forms a stable oxide layer that shields the metal. The odd thing is that iron starts out the same way: it is relatively unaffected by pure water or dry oxygen on their own, protected at first by its own tightly adhering oxide layer, a passivation layer much like aluminum's. But the moment both oxygen and water act on that layer together, it converts into rust, a reddish-brown compound made of hydrous iron oxides, and that rust is commonly flaky and crumbly, providing no real protection to the iron underneath. Instead of sealing the metal off, flaking rust just exposes fresh iron to attack, and given enough time, an entire iron mass can corrode all the way through.

Water turns out to be the real catalyst behind the whole process. Even iron and steel structures that look perfectly solid have microscopic pits and cracks that water molecules can slip into, and once chloride ions are added to the mix, as they are in saltwater, the corrosion only speeds up. Rust isn't even a single, simple substance: underwater rebar in concrete pillars, starved of oxygen, can form an entirely different compound called green rust, while a specific form called stable rust actually does protect iron, forming a thin protective coating, as long as it's kept away from moisture, even if it's still not quite as effective as the oxide layer that guards aluminum.

Far from being purely destructive, iron oxide is something humans have put to use since prehistory. Long before anyone understood chemistry, people in parts of Asia were painting cave walls with iron oxide pigment around 15,000 BC, with no binder at all, just the raw mineral itself providing the color. Today, iron oxides remain inexpensive, long-lasting pigments in paint, coatings, and colored concrete, and they're also used as iron ore, as catalysts, and in thermite reactions. Iron oxide even shows up inside living bodies: many organisms store iron as ferritin, essentially iron oxide wrapped safely in a protective protein shell.