Politicians reflect their voters' views. In most countries immigration rates are at least an order of magnitude too low to compensate for low fertility and will stay that way.
2. Most of the projected increase in global population is due to an expected increase in lifespan, meaning a greater number of old people. They don't have kids.
The big reservoirs of working-age people are in sub-Saharan Africa and rural India. Not many of them have the skills and funds to enable intercontinental migration. Yet.
Not sure why declining birth rate is a problem, though. Maybe we'll get around to automating some of the things.