Because, if we defeat them and leave them alone, they will regroup after 30 years and cause pain in our ass again. Until we exert control over provinces of Kashmir, Punjab, Sindh and Baloochistan we cannot eliminate the problem. Indian army needs to be deployed there for about 2-3 generations. At least 3 generations of children should be brought up under Indian nationalist educational system and religious values of completely Indianized version of Islam. These provinces, particularly Punjab should be kemalized, this can't be done without presence of Indian military. If this is done for 3-4 generations under vigilant Indian leadership which is not shy of expressing their Hindu identity, problems will start decreasing.
Furthermore, why should a western OR oriental power have presence in South asia in long term? It is better that we control that region.
If India takes them over, will they sit still?
Think that over.
Do you think the Indian Muslims will sit back and watch the show?
Add the population!
And if they start an insurrection in India, think of what you have bought for yourselves! What is happening in SWAT and FATA will be happening in India!
The Muslim mentality has to be understood. They are not sheep as some are here in this country. Give them their due. Even though they are a minority in India, they have ensured that their rights are not trampled and their separate identity maintained and respected by the others! Not a mean task, I would say.
Each one in Pakistan thinks he is a Khalifa and his sect or tribe is superior to all others! And since there are many faultlines in Pakistan, this could be nurtured and Pakistan will self destruct.
If I may remind, India is not a Hindu country! And currently, a Sikh and a Roman Catholic call the shots and so I daresay there is no vigilant Indian leadership which is not shy of expressing their Hindu identity as they are not Hindus!