And yes, there is in fact a 13th zodiac sign, Ophiuchus. The black and white reason tropical astrology doesn’t use Ophiuchus is because Tropical Astrology has always been seasonal. Instead, it uses exact 30° sections of the vernal equinox, decided after acknowledgment of Precessional Drift (which is also used to calculate an Astrological Age, and that’s how we know we’re still in the Age of Pisces) it makes astrology a little more mathematically* precise, i suppose, cleaner.
There are branches of astrology that use the actual location of the constellations, like Sidereal/Vedic/Jyotish adjacent, western sidereal, etc