Horoscopes themselves are obviously not always valid, yet they still fascinate us. I believe it's partially because they represent a set of archetypes that we use to identify ourselves and others. Consider:
Aries probably seems to represent the typical rash teenager, or old-western cowboy.
Taurus might be the typical businessman or member of the noblity.
Gemini could be the diplomat, lawyer, or socialite.
Cancer seems like a reclusive artist, or a very sensitive person with a lot of unexpressed dreams.
Leo could be a knight, or the visionary and exacting leader.
Virgo could be the dedicated, practical mother.
Libra might be an arbitrator or a philosopher.
Scorpio might be any fierce competitor.
Sagittarius could be an activist, possibly a hippie.
Capricorn might be the self-made wealthy individual.
Aquarius could be a scientist or researcher.
Pisces could be a typical melancholy dreamer.
So, the point is, I think people were partially attracted to these ideas as ways of creating archetypes by which to compare people. Perhaps it's even similar to what causes people to study psychology today. One thing I think we can gather from horoscopes, however, is a more holistic set of archetypes that explain how our minds natually categorize and set expectations for people. Does this make any sense?