1) With those billions, they'll make 10x as many, with updated technology, probably stolen from india, which the US is sharing tech with now.
2) The USA does NOT want to be seen stockpiling nukes .. it'll look bad, with the nuclear non-proliferation treaty & whatnot.
What the heck is the USA doing, giving money to pakistan. $3Billion could go to the red cross, or sponsor a crapload of more efficient charity foundations to wander into Pakistan and fix the damn country - most of the work is done by volunteers ...
It's a matter of bang for the buck.
$3billion dollars can: build 60,000 homes in pakistan ....
$3billion dollars can: line the pockets of corrupt Pakistani politicians, officials, police officers/army/terrorists and build 5 homes.