not every country can encourage significant immigration
even developing countries have a rapidly dropping birth rate
Some countries, maybe like Japan and South Korea, have low birth rates and a history of discouraging immigration. I’d argue it’s too late for them: you can’t suddenly develop and support a large wave of immigration, especially when most developing populations are doing better, most are seeing lower birth rates. They have a lot of work to do and little chance of succeeding
Other countries, notably China, have a rapidly declining birth and already see the impact, so are just going to discourage emigration. The supply of immigrants will quickly dry up (except refugees)
So for example, the US has a history of significant immigration. We’re already in the scenario of insufficient birth rate to sustain our population but sufficient immigration to keep growing. Maybe I don’t know enough about other countries or I’m falling to some sort of exceptionalism, but to me this boils down to why doesn’t US encourage immigration. We have the easy case: if we can’t figure it out, how can we expect anyone else to.