Let's talk about game systems and the fundamental assumptions they make society.
In the Traveller RPG (and the number-filed off version, Cepheus), planets have Populations and Governments. Populations are represented by orders of magnitude, and set at 2d6-2, leading to a Population score of 0–10, representing single persons to tens of billions. Which probably makes sense: an average number of planets have an average number of people (in this case, 105), and normally distributed around that. Realisticly I'd expect more of a long tail distribution more similar to city size distribution, but it's probably approximate enough.
In Cepheus, the Government score is a number from 0–15, where each number refers to an entry in a table. The Government score is 2d6-7, plus your population score. The table is reproduced below.
| Type | Description |
|---|---|
| 0 | None |
| 1 | Company/Corporation |
| 2 | Participating Democracy |
| 3 | Self-Perpetuating Oligarchy |
| 4 | Representative Democracy |
| 5 | Feudal Technocracy |
| 6 | Captive Government |
| 7 | Balkanization |
| 8 | Civil Service Bureaucracy |
| 9 | Impersonal Bureaucracy |
| 10 | Charismatic Dictator |
| 11 | Non-Charismatic Leader |
| 12 | Charismatic Oligarchy |
| 13 | Religious Dictatorship |
| 14 | Religious Autocracy |
| 15 | Totalitarian Oligarchy |
The table is biased with more authoritarian government styles at the higher numbers because the Law Level of the planet is calculated as 2d6-7 plus Government, but this incorporates the assumption that higher populations automatically result in more authoritarian governments. Whether or not this was a deliberate choice, or a concession to game mechanics, is unknown.
Also of interest is that the government score is for a World Government; Earth automatically falls under 7, Balkanization, as we still have multiple governments. The planets in these games generally assume single world governments, and surprisingly few of them give the citizens of these worlds any say in how they are run.
That being said, some of the governments that are not explicitly democratic may be bent towards democracy. Both Bureaucracies do not explicitly reject democracy, and under Balkanization some of the regional governments could possibly be democratic. Even the non-Charismatic leader might be democratically elected (we can hope), but the description in Traveller explicitly says the leader has replaced a previous dictator "through normal channels".
But no matter how we look, citizens across the galaxy have very little say in how they are run. The average government is a feudal technocracy, with some leaning to democracy and some taken over by higher powers, whereas a planet with Earth's population is most likely under the rule of a charismatic dictator.
What is this system modelling? Why is it making these assumptions? Some are standard scifi fare: world governments are assumed, and that Earth-like planets with multiple governments are an anomaly. But these systems are based of strongly military assumptions; character creation has The Draft and initially almost all the careers were militarily focused. Of course the governments would be totalitarian. The citizenry are allowed no say, these are dangerous times and only military might can guide us through these uncertain times.
These are the assumptions made by these systems.