President is only a notional head of Government in India. He actually has no original work to perform and he enjoys no original power.
When Shri Pranab Mukherjee was made President of India in 2012, it was not considered to be a promotion for him, since he as Finance Minister of India and the Head of Many GOM (Group of Minister) enjoyed far more powers and influence.
Another disadvantage of becoming a President is that you can’t hold any other posts in India after becoming President. You can’t be a PM, CM or a Minister after you had been President. Hence, becoming a President is practically the end of one’s political career.
Amit Shah is quite a young politician. He is just 52 years old and he can hope to be in active politics for next 20-30 years. He would any day prefer to be the Chief Minister of Gujarat, or a cabinet minister in the Central Government. He can also become a Prime Minister one day if he has a good luck.
He would never opt for the post of President even if it is offered to him on platter.