Why doen't Astrologer change their own future with the astrological knowledge ?

There is a very big misunderstanding in people regarding astrologers and doctors. They think the doctors cannot fall ill and astrologer do not have bad period in their life, because doctors are masters of medicines and astrologer are masters in remedies.

God has not given power to any human being ( accept Saints and Sadhus ) to alter his future. Future written by Vidhata at the time of birth is fix and final. The position of the nine planets in our kundli / horoscope decides speaks about our destiny. It is also said in some texts that our destiny in this life is due to our karma of our past life ( Previous birth ). We cannot alter it.

Astrology helps only in minimizing the negative effect of the planets but cannot completely remove it. It is applicable to an astrologer also, they are not above human race.
If the power of altering the future comes in human hand then people will start misusing it and thus will lead to destruction of the world.