Well, there are many motivations proponents of this can have. Some will cite high minded ideals of freedom of movement, world citizenry or the like. Some people think diversity is always a 'good', although the most generous societies tend to be homogenous, as people tend to not helping those they view as 'others'.

However, I think the real driver of this is the power desired by certain people to enforce agendas worldwide. Things like tackling environmental problems don't work unless everyone is doing it. We also saw this with covid policy, where countries that never agree on anything all of a sudden all agreed in lockstep to implement completely unfounded public policies like quarantining healthy people and shutting down economic activity.

The billionaires and corporations are so multinational, that the power structures between nations are now quite interelated. The corporations dont give two fucks about national borders, they are international profit machines that view themselves as stakeholders on a higher level than government