Didn't the Nightborne believe everything outside the Broken Isles had been destroyed by the Sundering?
Not sure if anyone but qualified militia, magi or elders were allowed outside the city. They had everything they needed in the form of arcwine and a harbor that, probably didn't allow seafaring?...
Again, I'm not sure. But if all this is added up, and you're practically living in your perfect bubble world, I don't really blame them for not wanting to leave, or being allowed to leave. US is the greatest country in the world right, why leave it?