First of all, Ukrainian experts who come to China are all experts in weapons, and most of them are experts in the research and development of large weapons. Aircraft carrier, large registered hovercraft, large military aircraft, large military ship engine, etc. , are military, but also large military equipment. At present, there are not many countries in the world that are still developing these large-scale military equipment. At present, Russia is powerless and has no funds. Although the United States has deep pockets, these military equipment and technologies are world-leading, and they despise Ukraine's second-rate technology. China is the only country that is still developing these weapons with equipment and technology.
Secondly, Japan, South Korea and Western European countries have no need to develop large-scale weapons and equipment. Japan, South Korea and Western European countries rely on big countries (mainly the United States) for national defense, so there is no need to develop these large-scale weapons and equipment. What they need is all kinds of civil science and technology, but these countries are themselves developed countries in civil science and technology, and Ukrainian technicians are very disdainful.
In other words, at present, there are about a dozen countries in the world that have the economic ability to build such equipment, but only China needs such equipment among these ten countries. So if these experts don't come to China, their skills will be useless.