Ptolemy's Almagest catalogues forty-eight constellations sorted into three lists, zodiacal, northern and southern, and the zodiac is the only one of the three defined by geometry rather than by story.
The twelve zodiacal constellations are grouped together because the ecliptic, the Sun's apparent yearly path, runs through all twelve and none of the other thirty-six, and the Moon and every naked-eye planet keep close to the same band, wandering only a few degrees to either side of it. That is a physical fact about the solar system's shared orbital plane, not a Greek invention: Babylonian astronomers had already fixed a closely related set of figures along the same band centuries before the Almagest, tracking planetary movement against them in the astronomical diaries that are among the oldest continuous scientific records to survive from antiquity.
The grouping's purpose in the Almagest was practical before it was symbolic. Ptolemy's star catalogue exists chiefly to support the planetary tables built around it, and a planet's position is only useful once stated against a fixed backdrop; the zodiac is that backdrop, the ruler the rest of the model is measured against, not a preferred set of shapes chosen for their stories. Ptolemy's later Tetrabiblos takes the same twelve and converts the ruler into meaning, dividing the band into twelve equal signs rather than twelve unequal star patterns.
That double life, ruler in one book and meaning in the other, is why Zodiac survives here as an entry of its own rather than dissolving into its twelve members: the set can only be read as a coordinate system, which is a different kind of claim about the sky than describing any one shape in it.