Opening borders would endanger those in richer countries
There are many criminals and terrorists in other countries who would move to richer countries if they could. Abolishing borders would threaten countries' national security.
Many poorer countries have much higher crime rates than richer countries. Some country governments are even ran by drug cartels! Letting those guys come into our country would be putting your kids at risk.
There are many terrorists and criminals living in poorer countries. If we open the borders they will come in and make our families unsafe.
Border controls do not necessarily prevent all national security threats.
People will take advantage of resources in richer countries if they can, even if it means infringing on those countries' national sovereignty.
[P1] Opening the borders will encourage terrorists and criminals to enter wealthier countries.