Why is Libra the only lifeless constellation in 12?

Not the only one. Libra and Aquarius are both. You can look at the name or design. Aries is a sheep, Taurus is a cow, Gemini is a man, cancer is a crab, lion is a lion, virgin is a man, and Libra is a scale. That is, the balance, which is a project. Scorpio is a scorpion, Capricorn is a ram, Sagittarius is a man, water bottle is a bottle, article, Pisces is a fish.