This is a common curiosity with no plain answer. Rationally speaking, you can only “believe” anything “perfect”. But the perfection does not exist in this world. Hence no worldly science or art is believable. No machine is perfect. No technology is error-free. No consultant is flawless. No expert is perfect. However, their imperfections and limitations are not generally questioned. On the other hand, we erroneously expect perfection from an astrologer. Like other professional arts and practices, astrological outcome is also subject to conditions and limitations. But they are not commonly accepted, because of astrologers’ tall claims, quackery in the name of astrology and people’s ignorance and superstition.