The immigration system should be designed to block retired parents from moving country to live with their working age adult children who have migrated.
One reason to have immigration is to improve a country's dependency ratio: the ratio of working age population to children and retirees.
The ideal immigrants are young well-educated parents that can stay in the workforce for 40+ years with healthy children that are just about to enter the school system.
That way the receiving country didn't need to invest in educating the parents originally, don't need to pay the healthcare costs of very young infants, and it provides the best possible addition to the the receiving country's demographic structure so the host country benefits from a whole working life of tax payments and all the value created by their work output.
The economic case for even skilled immigration is far less compelling for a receiving nation without such restrictions on immigrating retirees.