I have a question but why is every planet, a planet-state? culturally and politically?
Culturally it's almost never the case of having One Culture One Planet, very few planets have anything approaching a monoculture, and most of them are explicitly multicultural states. There are more languages spoken on Kahlo alone in the 28th century than in any one of the 21st century's most linguistically diverse countries. There are cultures that exist across multiple planets as well, including ones that have then subsequently branched as they have experienced a diaspora onto multiple other worlds, I've just not necessarily tended to dwell on them in focus yet, though I have talked about the Polynesians of the Stars as an example of a culture that is primarily based around nomadic starship-board lifestyles and not stationary/orbital existence. The Galilean Moons are a good example of emergent cultures of the solar system outside of the inner planets then spreading and intermixing, like how folks descended from Trojan asteroid dwellers/miners are common on Ganymede. The fact that many planets are named in specific languages might give the impression that these are planets of a single culture but it's not the case, they're simply often named by their earliest inhabitants or those that founded the planetary settlement, and reflect that initial cultural mix. This is just not a universe in which English names are, by default, neutral, including for planets.
Politically, however, single-planet states or nations are relatively common, it's a legacy of the earliest era of human extrasolar migration. With the demands placed on initial infrastructure and the establishment of suitable areas of habitation many of the planets began as a single area of urban settlement, with many of them still remaining focused on that urban area in the present day, making the vast majority of human settlement concentrated in a single area and relatively contained (though this is not true on a number of settled worlds and especially not on the oldest planets, where a much higher % of the surface will have a permanent human presence). Given that many of these planets were initially settled by specific and organised groups, most of whom had registered their intended planet and expedition with UNSA, that also tended to promote a single emergent state coming to encompass the entire planet, especially as the number of planets to go around meant there was no reason to attempt to crowd somebody else's planet in favour of one's own. In a universe where humans are not generally struggling for planets or, indeed, habitation, the occasions where mass migration does occur to an existing planet usually implies something more complex is going, and that they're actively seeking membership in a citizenship franchise. Other factors encouraging planet-states includes the need for diplomatic representation for a given planet's unique situation and requirements on the interstellar stage, both on the small scale and at the big stage, but also includes the simple practical fact that sharing orbital custody of an entire planet is super awkward. They really don't know how pre-FTL humanity managed it. The potential for conflicts regarding orbital control, or dominance of a planet's orbits in general, is another factor that encourages some form of single planetary authority, though that obviously doesn't preclude autonomy of components of the planet's human populations. Indeed, with a lot of worlds I assume a lot of relatively high local autonomy, to the point where the single planetary government primarily exists to co-ordinate orbital control, organise system defences, and negotiate with interstellar powers and organisations. However, I do still imagine there are planets with more than one sovereign state, even if they're rare, and I do plan to cover at least somewhere that functions like that.
That's the explanation for why relatively few planets are home to multiple sovereign states, but there's a separate reason for the reverse, i.e why almost no states encompass multiple worlds. The UN became involved in extrasolar peacekeeping relatively early on in the Crumpleverse's history due to Silly Things being attempted by a number of the early space powers. They made it clear that they did not look kindly on warfare between planets for territory and profit, nor forcing a colony founded by your citizens to be subordinate, nor the forcible annexation of other planets or any other means of forming interstellar Empires. If you wanted to claim otherwise, from the late 22nd through to the late 25th centuries you would be dealing with the single largest spaceborne military force in the human sphere, until UNSA collapsed to be replaced with ISA in the mid 26th century. However, the interregnum period saw a lot of multi-planet alliances emerge for self defence, many of whom are still operational and somewhat function like multi-planet states for the purposes of foreign affairs and defence. The Xia-Liao pact is one such that has made an appearance flag-wise, which is an alliance between Hra (shown on this round of flags) and Minhia (shown on the 3rd set). An exception to the rule that did not originate in that period as an interplanetary defensive/protective alliance was India, which has existed as a federal interstellar state for many centuries. 'India' consists officially of India on Earth and three other planets, with a shared fleet apparatus and other shared franchises besides. It managed this where so many other early space powers did not because it never leveraged any kind of imperial control over its component members nor seek to use this multi-planet scale to promote militarism or interstellar aggression. The Galilean Republics are also an explicitly multi-moon state, though on a day to day level they have separate local governance. It's just that anything that remotely resembles a proto-Empire or an aggressive move in foreign policy is going to get you stared at very uncomfortably until you stop.