Here's why:

* They are based on Earth's position: Zodiac signs are defined by the constellation the sun appears to be in from Earth's perspective at the time of a person's birth. However, the constellations are not fixed, and our view of them changes as Earth orbits the Sun.

* The constellations are not equal: The constellations vary in size and shape, meaning that the time spent "in" each constellation is not equal.

* The Earth's axis precesses: Over thousands of years, the Earth's axis of rotation shifts slightly, meaning that the constellations the sun appears in during a person's birth would be different than thousands of years ago. This shift is called the precession of the equinoxes.

The historical context:

The zodiac system originated in ancient Mesopotamia and Babylon. They were used for tracking the seasons, understanding celestial cycles, and making agricultural predictions. It was not a system based on personality traits or destiny.

Modern astrology:

Modern astrology uses the zodiac signs as a framework for understanding personality traits, compatibility, and predicting future events. However, this is based on belief and interpretation rather than scientific evidence.

Conclusion:

While the zodiac signs hold cultural and historical significance, they have no scientific basis for determining personality or future events. It's important to remember that astrology is a belief system, not a scientific theory.