The USA does not want its weapons to be used to attack Russia. No problems if Ukraine buys or manufacture it’s own weapons and uses them. Also there maybe concern that if Ukraine uses long range weapons, Russia may declare a ceasefire and if Ukraine continues to attack russia will invoke security articles under the CSTO and possibly security agreements under the Shanghai cooperation agreement. So there is possibility that if USA sends the missiles to attack Russian territory then the other nations in central and South Asia maybe required to help Russia defend against the attack in the same way nato is helping Ukraine. The USA has seen this happen before in the Korea war, Vietnam war and they saw the shots how that followed the collapse of the iraq government in 2003.