Or is it rather the way it is because people have actually liked to engage in more transnational relationships of all kinds, commerce, communications, cultural exchange, migration, educations, etc., and it turns out that having an effective political organization with the power to establish and maintain institutions facilitates transnational living?

well, it is rapidly appearing that 'having an effective political organization with the power to establish and maintain institutions' is more figment of imagination, wishful thinking and historical nostalgia than reality, except the pentagon, and even that looks a lot less effective lately.
santiago:

if people have what they need for a happy life at home ...

That seems like a pretty big "if" to me.
 and maybe it's those people happen to running things now more than others.

heh, i should have said 'happi_er_'.

santiago:

if people have what they need for a happy life at home ...

That seems like a pretty big "if" to me. Maybe there will always be some people who aren't happy with life at home, and maybe it's those people happen to running things now more than others.

with all we know these days about which essentials/baubles, intellectual or material, make people happy, (oops happier), that's why we need to do that.

were a millionth of the energy put into financial profit invested in more humanity-enhancing activities, we'd outgrow our collective, infantile need for hegemons. indeed america might have already fulfilled  the important transnational part of its destiny, and now like a dandelion head, can pass into history, knowing the seeds of its better ideas are winging everywhere the winds of technology blow.

ideas and technology outlive the empires that spawn them...

'The history of public debt is full of irony. It rarely follows our ideas of order and justice.' Thomas Piketty