Why should immigrant countries restrict studying abroad for the purpose of immigration?

1. The population of China is too large to burn. And no matter how many immigrants come in, they are not immigrant countries because the local national base is too large. China people find it difficult to find jobs, even for their own people. If they emigrate to high-end talents abroad, they will definitely supplement China's strength. For example, when the Soviet Union disintegrated, a large number of CIS scientists went to China to work and earn a living, which greatly promoted the development of science and technology in China. But these are a minority after all, and more are low-end people. There is no shortage of this China, and I want to export it. 3. On the other hand, in Europe and America, there are many high-end talents and few low-end talents, and they need low-end people to provide services for them. Of course, they also need high-end people, but there are more low-end people. Such as nursing, picking up garbage, etc. What these locals don't want to do is done by outsiders.