Why "curiosity" and not "cury"?

I've been looking at the adjectives "curious" and "furious", and have been wondering why their noun counterparts are so different.

According to etymonline, the origins of these words are:

"furious" comes from French "furieux" from Old French "furios" from Latin "furiosus" ("full of rage, mad"; related to "furia" ("rage, passion").

"curious" comes from French "curieux" from Old French "curios" from Latin "curiosus" ("careful, diligent"); related to "cura" ("care").

Now, why is it that -- despite these two words having a similar structure at each point in their history -- their English nouns are so different, when their adjectives are so similar?

Why do we describe people as "curious" and "furious" but call these attributes "curiosity" and "fury"? Why not "cury" and "fury"? Why not "curiosity" and "furiosity"?

At what point in the evolution of these nouns did they start to diverge, and why?

Many adjectives end with the letters ...ious. From the list below (taken from Spellzone) I can only see three which support nouns like fury - melodious (melody), harmonious (harmony), and victorious (victory). The rest have nouns that are connected in a variety of ways, some with no noun connection at all.

amphibious (amphibiousness?), curious (curiosity), delirious (delirium), devious (deviousness), dubious (dubiety), furious (fury), harmonious (harmony), impervious (imperviousness), melodious (melody), notorious (notoriety), oblivious (oblivion), obvious (obviousness), precarious (precariousness), previous (none), rebellious (rebelliousness), serious (seriousness), studious (study - a verb), tedious (tedium), various (variety), victorious (victory, victoriousness)

As regards the two you mention, whilst their etymologies are both French, as @sumelic has pointed out, they were treated quite differently to one another in Latin. That is perhaps relevant. The OED says, in respect of each:

Etymology: < Old French curius ( Ch. de Rol., 11th cent.) = Provençal curios, Spanish curioso, Italian curioso < Latin cūriōsus used only subjectively ‘full of care or pains, careful, assiduous, inquisitive’; French has also the objective sense in 14th cent. (robes curieuses). A word which has been used from time to time with many shades of meaning; the only senses now really current are 5, 16, and (in some applications) 9.

Etymology: < Old French furieus (modern French furieux ), < Latin furiōsus , < furia fury n.