Question

why is ammonium nitrate not used to produce ammonia

Answer 1

Because the gas produced is very harmful

Answer 2

here is your answer
The industrial production of ammonium nitrate entails the acid-base reaction of ammonia with nitric.Ammonia is used in its anhydrous form (i.e., gas form) and the nitric acid is concentrated. ... The ammonia required for this process is obtained by the Haber process from nitrogen and hydrogen.