#Make Building Motivation Part of Campaign Setup
Make Building Motivation Part of Campaign Setup
The easiest way to make sure the PCs have things to do in the setting is to create the characters and the setting together. That way you know there'll be relevant stuff for the PCs to do, since you are creating both elements together with the express purpose of making them interlock.
A great time to do this is right after Same Page Tool: the answers about party cohesion and player/GM roles will guide what choices for situation and PC goals are appropriate for what you want to get out of your game. (Some examples: if the characters are 100% working together, it's important to give them shared goals and make their personalities compatible; if you want the game to be mostly PC-driven, then characters need strong — maybe even slightly ridiculous — goals to consistently spur them to action.)
Note that this doesn't mean the players have to write large chunks of the setting themselves (though it's fine if they do). It's more like you're making sure the characters are primed to be the protagonist of the story. For example, if someone wants to play an impoverished knight trying to reclaim his birthright, it's certainly helpful to establish what his birthright actually is and how he intends to go about reclaiming it, so that you can actually incorporate those elements into the game. Where this differs from just "writing a backstory" is that you're explicitly establishing the focus of play. It's completely fine — I'd argue good, even — if the characters don't have actual backstories; what matters is their present and their path forward.
If your players can't figure out how to write self-motivated characters, ask them questions about what they want to see in the game and use that to guide them in writing their characters to pursue those things.
If your players having trouble processing the idea of their characters having their own "proactive" goals, it may be a sign they really want strong guidance from the GM during play. If that's the case, straight-up "sandbox" play won't work for them.
Then, Start in Motion
Imagine the pilot of a TV show: bringing the main characters together, introducing some antagonists, establishing the big issues and struggles of the show, et cetera. Now, don't play that.
Instead, make your first session more like the second episode of that show. Everyone already knows each other (how is established during character creation). They're pursuing some common purpose (also established during character creation).