But even when making those improvements, missiles of a country are not compared with missiles of other country. If a missile takes off and hit its target within margin of error, it is regarded as a success. No one cares and cribs that the missile is less fuel efficient than an American missile or that it carries only 3 MIRV while American/Russian carry 5; but in case of a jet Engine, you need to compare it with those available in market ,and even if you are ready to accept some loss in capability, it could not be grossly inferior to those available in market, else your aircraft would be under-powered thus compromising whole aircraft.
Making missiles and rockets is like sitting in boards exam where you have to get more than passing marks, while making an engine is equivalent to sitting in a competitive exam where you need to be better than your competition.This is the reason as to why it is easier to "succeed" in building rockets and missiles than engines.