Pakistan is 340,509 square miles in area. A 20 kiloton bomb such as those dropped on Japan during WWII destroyed a 10 square mile area in each city. In order to "glass" Pakistan, it would require 34,051 of the bombs dropped during WWII.
Do we have bigger bombs now? Yes, we do, but we simply don't have enough in stock to do the job. On top of that, the larger the bomb, the higher the mushroom cloud goes into the air, and the further the fallout will spread. There is a reason (other than Cold War reasons) that no country, no matter how angry or bloodthirsty, has attempted such a thing after WWII. The price tag for total nuclear annihilation is simply too high, in both dollars and in worldwide environmental consequences. Regular bombs such as incendiary bombs are much cheaper and have less of a detrimental effect on surrounding countries, so that is why they're still our go-to bombs in modern warfare. It would still be very expensive and impractical to "glass" even the smaller countries like Pakistan with incendiary bombs. That's why it hasn't happened already.