Take the Dune books. Ostensibly the original book (the only one that matters) takes place on one planet and in one city on the planet. The star-spanning cultures of the Dune universe are only inferred through the thoughts and actions of the characters. Neat trick I say. So Frank Herbert created a huge galactic society by not creating a whole huge galactic society...
It begs the question how much world-building should a referee do at the outset of a new campaign? And it seems not much. I appreciate the brutal nakedness of the first generation of roleplaying games. Here is a set of rules tilted towards an adventure genre so when you create your own classic vision of sci-fi, western, fantasy the rules will support the referee's efforts. The first part of the original rules for Traveller accommodate this game philosophy through character creation. The method is such a neat "trick" players and referee can begin a game with little prep and plop media res at opening scene. Something as simple as "You are in the starport bar when a stranger approaches you with a proposition." Now players are sure to start squawking for setting information; what bar, what planet, what system... What navy, army, scout service spawned my character?
I think the nimble referee looking to build a game universe around their player's characters is well rewarded by utilizing Classic Traveller rules. It is awesome if the referee has a crystal-clear idea on what the world setting will ultimately be about (Dune is a good example). Players get to "grow-up" with the game universe and learn about it like you would in real life, through experience. But if not, the game still supports the referee through all the important steps of adventure creation and campaigning. Without resorting to a pre-built universe to show you "how it is done."
In conclusion, Traveller was once able to assist you with whatever sci-fi subgenre tickles your fancy. Planetary Romance, Hard Science Military, galactic savants and sentient planets, telepathic whales and rabbit-holes of new discoveries. Demons, wormhole passages, dreamy natives living on top of the ruins of ancients. Its use was quickly blasted away under the understandable need for gamers to be given a starting point, an official universe and the understandable need for the company to sell what the majority of gamers want.
Long and short of it, I'm a relict of gamings past. The original design philosophy of the likes of Arneson and Miller leave me not wanting much more from the company outside of their genre specific rules. It is a concept I can lose hold of in the product push by game companies trying to pay the bills. Unfortunately for game companies fierce creatives will use their rules well, but not drop much on additional merch.