It doesn't have to; the deeper the rebar the longer it will last. Rust takes more space than iron, so it can't form if the integrity of the concrete is uncompromised. You have to weigh the extra concrete against the likelihood the structure will be obsolete anyway. Building a 4-lane floating bridge across Lake Washington that drains runoff directly to the lake wouldn't have mattered if it would have lasted longer because it was too small and environmentally damaging in other ways.
