This is a common misconception. An aluminium or cf fame with the same strength as steel is often heavier. The differences are that aluminium is cheaper to tig weld in mass production than steel, hence its popularity in cheap frames whereas with cf, you can build in the strength only where you need it along with greater flexibility in cross section, thus leading to weight savings in the MORE EXPENSIVE frames. Real cheap cf frames may not offer anything. A manufacturer who is carefully choosing his cro-molly tubing for the job can produce a much better fame for the same weight and cost than ally or cf.
As prices go up, the equations change dramatically.