Ebro38,
Yes, quite correct. If the stress in the steel is below the endurance limit it will "never" fail in fatigue. Exactly the same is true for aluminium. Steel "generally" has a higher endurance limit than aluminium.
I don't really know what the stress is in a bike frame, but I'm guessing that it's well below the endurance limit for steel, but may be slightly above the endurance limit for aluminium. Which is why you
may see fatigue failure in an aluminium frame compared with a steel frame.