Because it is not a zodiac sign but a constellation.
The difference lies in the fact that the zodiac signs arise from a duodenary subdivision of the ecliptic, the sun’s apparent path throughout the year as seen from the Earth. The constellations, or the drawing of lines to unite the visible stars to form recognisable figures, came to light in the Egyptian era for mnemonic purposes, to fix the times of sowing, harvesting, and flooding the Nile based on the heliacal rising  of the stars.
Therefore, the western zodiac is on a seasonal basis, originating from a division for twelve of the sky, in analogy to solar irradiation at various times of the year. Due to the precessional motion of the earth’s orbital axis, the constellations of reference move by one degree every 72 years, so today they are out of phase concerning the zodiac signs; but this does not in the most minor influence the symbolic structure of the tropical zodiac.