Immigration is not a static phenomenon. The birthrates of immigrant groups drop to more closely match the existing population, and the origin nations of immigrant groups change over time, sometimes eventually becoming nations with net positive immigration. Colombia for example was a nation with net negative migration due to the drug war and political violence, but is now about even due to a surge of immigrants from Venezuela and the US. Or just think about the patterns of Irish or Italian immigration to the US.

There's also a huge historic and geographic factor, specifically colonialism, that creates connections for immigration that would otherwise be unlikely. It's not like this is some clockwork system where every citizen migrates to the richest country that can reach at any given time. It's people, by the tens of millions, making the best decisions they can for themselves and their family based on the political and economic realities.