It is done because India is democratic country and all decision are taken by Bureaucrats, Ministers, Various departments and Experts in Defence field. It's still good but one thing is extremely bad which is UNLIMITED DELAY
1st - IA or IAF or IN send proposal to MoD as per their requirement. Then few experienced people in MoD do R&D on that, Then if needed then they send the proposal to MoF and few other department, etc., If all passed Then tender comes, Then they short-list, then many trials and then by seeing 3 factors 1> Price 2>Need 3> Diplomacy - They Purchase. In all these process it takes long time and sometime even half decade also.
On the military knowledge of politicians: "I wonder whether those of our political masters who have been put in charge of the defence of the country can distinguish a mortar from a motor; a gun from a howitzer; a guerrilla from a gorilla, although a great many resemble the latter." [15]
But, I think during defence purchase specialists from Army are included in the team...