Large-scale space fiction returns again and again to empires, federations and colonial frontiers. The pattern follows from the physical constraints a writer accepts when the setting spans star systems.
Distance creates the governing problem
Any story crossing interstellar space has to decide how information travels, and every answer creates delay between a decision made at the center and its effect at the edge.
Delay is exactly the condition that produced historical empires, where a governor acted on months-old instructions and the capital learned the outcome much later.
So a writer who takes the distance seriously arrives at an imperial structure almost by default, because nothing else survives the communication lag.
Autonomy at the edge generates plot
A frontier settlement that cannot wait for authorization must improvise, and improvisation by people with incomplete instructions is the engine of narrative conflict.
The center then has to decide whether to ratify or punish what was done in its name, which produces the political tension that carries a long series.
Writers get this structure for free once they accept the distances, which is part of why the shape recurs across otherwise very different books.
American readers bring a frontier frame
Much of the genre's commercial center is in the United States, and the national story of continental expansion supplies a ready set of images for settlement and remoteness.
Those images arrive with their own assumptions about empty land, self-reliance and distant government, and writers have inherited them along with the setting.
More recent work argues with that inheritance directly, treating the colonized side of the arrangement as the subject rather than the background.
Scale requires a unit of organization
A story spanning many worlds needs some structure that makes those worlds legible to the reader, and a political order does that work in a single word.
Naming an empire tells a reader who holds power, what the stakes are, and which side a character is on without pages of explanation.
Alternatives exist, including trade networks, corporate charters and loose confederations, but each is a variation on the same problem of coordinating across distance.
The form allows the argument to change
Because the imperial frame is shared, a writer can assume the reader knows it and spend their pages disputing it rather than establishing it.
That is why the subgenre keeps producing books that are recognizably space opera while reaching opposite conclusions about who the story belongs to.
The structure persists because it is useful, not because the genre agrees about what it means.