parts of speech - Why is "subconscious" used as a noun, while "conscious" is not?

subconscious and subconsciousness
conscious and consciousness.

While each one has a noun counterpart that is explicitly a noun, why is it that only subconscious is also used as a noun while conscious is not?

Some background info on subconscious and subconsciousness:

Subconscious vs subconsciousness

TLDR: subconscious and subconsciousness have slightly different meanings, even both as nouns. But that still begs the question, "Why not the symmetrical counterpart to conscious and consciousness

I'm going to give the benefit of the doubt and ask the question openly and give it all objective reasoning, but my most probable hypothesis is that English & people are quite asystematic, and this is just an ad hoc clusterfrack phenomenon of language evolving without thought... or subconsciously, if you'll permit my bastardization of that term.

Answer

Conscious is used as a noun:

In psychoanalysis, the component of waking awareness perceptible by a person at any given instant; consciousness.

(American Heritage)

As an example, from Contemporary Theories and Systems in Psychology by B B Wolman:

The conscious in Jung's theory plays a secondary role as compared to the unconscious.