It's self-evident no one owns the land and that each human is just passing through, much as each plant or animal.

For convenience sake, at this time, we have the world broken down into nations which gathered people who looked a certain way and spoke the same language.

Not surprisingly, as the masses move around and cultural diversity takes root in each land the national fabric is torn asunder from inside. Europe is a grand example of this, and the US is exhibit B. Nations are torn asunder by individuals with their own needs and desires, seeking joining with others from where they originated.

Sharia law in the UK is one such threat from within to that nation and every other nation. Whe one's nation is inhabited with sufficient numbers who do not identify with your nation, that nation is soon threatened with non-existence.

So highly restrictive immigration policies strengthen a nation. Japan is a great example of this nationalist xenophobism. Such a nation remains small but fiercely loyal to the nation.

In the US we have millions of folks whose allegiance is to Mexico and who would proudly wave the Mexican flag over the US one, even though that nation and its policies has impoverished them.

If a socialist or communist nation is given enough power they will seek to absorb their neighbors eventually seeking only one nation with omnipotent power over each human and control of the planet. Naturally, an intelligent person realizes such nations are undesirable because they limit human freedom. These nations value not the individual but the collective and freedom is the first thing to go out the window in a collective.

A diverse grouping of competing nations gives individuals more personal freedoms. In the 21st century, much like corporations, these nations must accomodate the will of their citizens to some degree or risk disaffection and dissertion to other nations. The iron fist communist nations have had the highest dissertion rates and the people, at their first safe opportunity, dissolved these nations.

Planets are widely spaced apart and are plentiful. They are also highly specialized and unique. The needs of one sentient organism will be geared to the design of their homeworld. Ohter worlds will not be nearly as attractive and the technological threshold to reach and survive in these other worlds is significant enough to corral most sentient races into their own solar system. Those that can safely cover the distances are evolved enough to create their own worlds and shape energy as needed. Planets no longer are needed for raw materials or "space" and the conquest stage is safely behind them, supplanted by a vastly superior technocracy.

Controlling and mastering the manipulation of energy supplants controlling land, "nations" or planets.