A question (which if you've already answered upthread, sorry!):
What are you trying to achieve? A sandbox in the (semi-literal) sense of the players taking their PCs around the gameworld exploring it? Or a player-driven game, where the key choices and direction come from the players as much as or more than from the GM?
If the former, I can't offer much advice as I don't run those sorts of sandboxes, and the few times I've tried they haven't really worked. For the latter, does it matter if the players have their PCs wait around for the adventure to come to them? The key is that, when it does, they choose how to respond to it.
Yes, player driven is what I would like. Let me explain my approach and my observations in a bit more detail...
At the start of his campaign I tried to get deeper characters but to no avail. I think one player really went for it, the rest didn't go much deeper than their class. I encouraged them to develop inter-party connections but also to little avail. When I asked about the type of game they wanted, I got a lot of "whatever's", "a balanced game", "we like how you DM", and one player specifically asked for a "heist adventure".
I decided to read in between the lines, taking what I knew about each player and their PC and work that into the story with a red dragon attack the PCs would need to react to. When it comes to the local scale, within the context of an adventure I explicitly introduce, they enjoy getting strategic and are good at coming up with creative ideas.
It's at the larger scale, choosing between different adventures, deciding on a broad course of action, or generating quests of their own that they forget critical information, suffer analysis paralysis, or fall silent and wait for me to give them some guidance. That's what I find not fun.
I have a fairly robust game world with lots of encounter possibilities, and I also have an overarching story involving an invasion of the PCs' kingdom by a magocracy. I have divided the story into separate adventures that are self-contained, can be pursued in whatever order the players like, can be approached in multiple ways, and each have multiple end states and can influence each other.
IOW I like a fairly involved complex game world, but my players have trouble keeping up because they have more casual attitudes toward the game & because they've been "institutionalized" to follow the rails.
Following the rails may not be a problem *for them*, but it is a problem *for me*. As a DM I put work into a game because I want to see players engaging with the setting and NPCs, and when I don't see that I feel frustrated. Why should I put work into a game if I'm not getting met halfway? The answer of course is: Because I love D&D, but I need more than that to stay happy DMing this group.