All the other signs are constellations as well
No they're not.
The signs are sections of the ecliptic (the sun's apparent path around the earth) measured from the solstice and equinox points. By definition, the beginning of Aries is where the sun is at the March equinox, the beginning of Cancer is where the sun is at the June solstice, the beginning of Libra is where the sun is at the September equinox, and the beginning of Capricorn is where the sun is at the December solstice.
That divides the ecliptic into four sections of 90 degrees each. From there, each section is divided into three signs of 30 degrees each. That adds Taurus and Gemini to the spring quadrant, Leo and Virgo to the summer quadrant, Scorpio and Sagittarius to the autumn quadrant, and Aquarius and Pisces to the winter quadrant. (For convenience, that's based on northern hemisphere seasons; it is, of course, reversed in the southern hemisphere.)
None of that has anything to do with constellations. It's simply a mathematical breakdown of the ecliptic.
But to muddy the waters, each sign of the zodiac shares its name with a constellation, and some 2,000-3,000 years ago, the constellations did align more or less with their namesake signs. Never perfectly, because none of those constellations are exactly 30 degrees; some are much smaller and some are bigger, but close enough to sync them for astrologers.
Thanks to precession, the constellations no longer align as closely with their namesake signs. Most of Aries aligns with the constellation Pisces instead; most of Taurus with the constellation Aries, and so on. But because the signs are not constellations, they don't move when the constellations move. And because the signs are mathematically calculated based on the ecliptic, and are by definition 30 degrees, there's no way any more or less than twelve of them can exist. A circle is 360 degrees, by definition; you can't divide 360 by 30 and get any number but 12.
