Why should god not be called father?

God can defiantly be called father because he is our father. Not our earthly father but our heavenly father.

If you believe in god then it may not be a good idea to call him father defiantly. He may take that as a challenge to his authority.

The Chapter of Unity IN the name of the merciful and compassionate God. 1 Say, 'He is God alone! 2 God the Eternal! 3 He begets not and is not begotten! 4 Nor is there like unto Him any one!'

So the answer, you can not call God a father even in spiritual term.