There are several routes to legal immigration. You are describing one of them. There are green card slots for: skilled workers, unskilled workers, family reunification, refugees, visa lottery winners and I think very rich people like George Soros.
By far the largest category is family reunification. Once one immigrant becomes a citizen he can petition to bring his parents, spouse, children, brothers, sisters, aunts and uncles. And as each of these become citizens they can likewise petition. We end up with chain migration where we import whole villages and tribes and a lot of them are older folks and unskilled folks who represent a taxpayer burden.
This area of our legal immigration policy badly needs reform. We should make family reunification easier, faster and in fact almost automatic but it should be limited strictly to spouse and minor children. Once somebody is a citizen they can always go visit their parents and extended family or have them visit America but the taxpayer should not get stuck with welfare for a bunch of old people and ignorant people who have never contributed anything to the tax base.