Super glue (”instant glue,” cyanoacrylate glue) doesn’t actually dry, it cures –and it cures only in the presence of moisture (which is present in the air). So super glues aren’t “sticky” themselves like “white glues” (Elmer’s, etc.) or animal glues are, for example, but they are highly adhesive on certain types of surfaces when two exactly-matching surfaces are pressed together in the presence of moisture.
If the cap is left off long enough, the moisture in the air will eventually begin to cause the glue to harden.