Probably because that, as I mentioned earlier, is akin to requiring your characters to be entrepreneurs. In the real world, relatively few people have the personality to be entrepreneurs. Why would these same people want to do so in game, suddenly?
Relatively few people in the real world have the personality to be assassins, or pit fighters, or holy warriors, or make deals with demons in exchange for supernatural power, but they readily assume these roles in roleplaying games.
How is assuming the role of a proactive adventure-seeker any different?
Probably because to a significant number of players that doesn't sound very interesting. They'd rather have an adventure semi-spoonfed to them that's exciting than go look for a 9-5 in-game.
That's actually my point: most adventurers aren't looking for 9-5 jobs, so in the games I run that means they should be actively looking for adventure, stirring the pot, making contacts, gathering intelligence.
The spoon-feeding part is, in my humble opnion, simple conditioning. It was not always thus, and it need not be so everafter. If more referees stop spoon-feeding their players, if more players assume greater responsibility for what happens in the game, I think you'd see a sea-change in how published adventures are written.
How is this different from how I run a game, where I throw out all kinds of potential hooks for players to do with as they will? Other than that I "railroad" a bit for the first session or two until the players have found their own feet, and when I'm ready to stop, I start "forcing" things to come to a head and close out satisfyingly, what you describe as your sandbox sounds nearly exactly the same as how I describe my non-sandbox.
Well, railroading the first session or two and forcing things to reach a conclusion are pretty significant differences.
But those differences asde, perhaps the key difference is none of what I'm putting out to the players are "hooks." There's no 'line' attached, no adventure path or series of linked encounters to follow, no BBEG encounter to be "forced" in for a "satisfying" conclusion.
In planning my game, I begin with a beginning, and then I full-stop. I don't give much thought to what happens next, because I prefer to take my cues from the adventurers. Their choices, leavened by fate in the form of dice rolls, make up the game.
For example, let's say the adventurers in
Le Ballet . . . take a stroll on the Pont-Neuf. I'll decribe the basic setting -the merchants and their stalls, the actors busking, beggars, well-dressed bourgeoisie and nobles strolling in their finery, the archers of the provost-marshal on patrol, students in their gowns expounding on some point of theology in Latin, the shouting bargemen from the river below the bridge. And then I'll roll for random encounters: maybe it's a beautiful noblewoman and her maidservant, or a pickpocket masquerading as a beggar, or students from rival fencing fraternities trading insults and looking fierce. I'll throw in a random reaction roll should the adventurers interact with any of these, to guide me on the npcs' behavior.
And that's where I stop. Everything else follows from what the adventurers choose to pursue. I don't prepare plots for the adventurers to chase; I assess the consequences of the adventurers' actions, and the world responds accordingly.