Because astrology has no scientific basis of fact. It is not testable, there is no modified evidence when things are proven to be wrong. When a scientific "fact" is proven to be wrong, the new evidence replaces the old. This is why I think that most people think of astrology as a fun part of the morning paper read to see what kind of day they will have. Rational people can't possibly believe in astrology, so no, I personally do not believe in it, nor anything else that is not based on evidence and is testable for proof.
Science is constantly testing and looking for new evidence. Even long held theories and facts are subject to change if new evidence is found. Astrologers do not apply the same rigor and are more accepting. In other words, astrology is not based on the same depth of testing that science expects and is therefore considered "just predication and zodiac signs".
Even though some astrologers spend a great deal of time trying to convince people that there is some connection between astrology and the natural world, astrology is basically a belief system and pseudoscience.