Sir we lack the requisite skills tech and infrastructure. And a matter of policy. Though it is not that we have not learnt from or reverse engineered any tech. US doesn't even authorise its weapons to be used at times Russians, French and Israelis are way more liberal than that. Russia has been more the liberal with us than any other, sharing top-secret tech which took decades in r&d and billions in investment, is not what every one is comfortable with always. Again at times it is less about ToT and more about our ability to assimilate & utilize that tech.