I would argue that this is not really true. You can't really understand the ramifications of theoretical political systems with thought experiments in a book or movie for because political systems are complex and ramifications thereof are harder to predict than the stock market.

A lot of sci-fi instead seems to recontextualize common human issues into an environment where existing political pre-conceptions can be avoided. Obvious examples are things like the half black/half white episode[1] of Star Trek TOS or "Measure of a Man"[2] from Star Trek TNG. These are pretty highly regarded episodes and neither really has anything to do with the Federation of Planets or its political system (in fact there are very few episodes that deal with these topics). Instead they're explorations of racism and what it means to be human/sentient.

[1] https://en.wikipedia.org/wiki/Let_That_Be_Your_Last_Battlefi...

[2] https://en.wikipedia.org/wiki/The_Measure_of_a_Man_(Star_Tre...