The main issue is that the entirety of the planet is now claimed with delineations (property lines) of private and national concern. As such, in order to migrate somewhere, someone else who is already there either has to move or share their land. In the past when this has happened it resulted in warfare. (excellent examples being the germanic migrations into the english islands in the 400-600AD by saxons displacing the welsh and engles and then again by the danes/others in the 800-1000AD)
But back then, borders were very porous, squishy things. Most nations now have legitimate fences and walls between others (borders like the usa which have few active defenses are incredibly uncommon elsewhere) and when caught entering without permission, you face imprisonment/eviction.
Whereas when you used to migrate, people would basically either displace the current residents of a land, or fill in the gaps. Now, you need to buy those gaps and typically those fleeing places aren’t exactly wealthy.
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