You are seeking a rigid scientific answer and you will not get it. People have actually answered this query you present and you fail to receive it as answer because you have predetermined idea of the answer you wish to hear. Astrology is not a hard science is the answer. hence if you accept this; you are also accepting the notion that science will never be able to approve or disapprove astrology. It's like asking why does the Moon wane and wax? and why do people seem to be more tense, emotionally charged, or even feeling full of life or full of anything when the Moon is full? And why do people seem to end things, move or say goodbye when the Moon is waning?
Wouldn't you say then that astrology is more akin to religion than say sociology?