It is.I guess some DMs like to be the giant watchmaker in the sky. Set the sandbox in motion and let the chips fall where they may. The prep effort, though, just seems immense…
The games that come closest to this breadth of sandbox style are computer games. And even they have rails for individual missions. Some are far more obvious than others. I've been playing Grand Theft Auto V recently, and am amazed at how little I can deviate in a mission. Yeah, GTA-V may have all these sandbox elements, but the moment you try to get onto the story, the rails grab you.
If you want a broad sandbox, it is pretty certain you'll need to improvise encounters during play. Random Encounter tables aid this by providing limits on what you can find. So things make sense. And, honestly, can lead to amazing play. Or awful play.
I am not personally fond of the "I move this encounter to where it makes sense" mode of play. Despite improvising encounters all the time. I like the sense of there being a defined world that the players can explore, even if it has undefined elements that are expressed through random tables.
I'm just horrible at prepping for the sessions these days.
I'm pretty convinced that there are a sizable chunk of players who LIKE defined stories that they play through. I think the Dragonlance adventures, if released today, would do better in a world used to the idea of story-arc D&D rather when they were released. Some of the adventures don't hold up and need reworking, but others I think are fascinating. (Sales of Pathfinder APs indicate that the form has legs. Consider that Paizo mostly discontinued their one-shot line of adventures).
And a major part of the reason for these adventures is to reduce the burden on DMs. Creating an entire living, breathing world is a lot of work.
One of the things I have liked about the current range of D&D adventures is that Wizards have experimented with the form. The adventures are not all alike. I pay a lot of attention to the structure of adventures, and they keep trying new things. Not all the experiments are successful (Descent into Avernus is the one I really dislike), but enough are.
And not all the adventures are for everyone. I personally think Tyranny of Dragons is one of the top adventures of all time for D&D. I've run it three times, and it's been an amazing success each time. But I know that many DMs struggle with it and dislike it. I'm lukewarm on Tomb of Annihilation, but I have friends who absolutely adore it.
Of the current adventures, I'd say that Rime of the Frostmaiden is the closest to pure sandbox, even though it has scripted elements. And it has all the challenges for the DM that a sandbox has: lots of details to keep track of, and it works best when the DM is proactively using the villains. (Princes of the Apocalypse would be similar).
I mean - it's 320 pages. Yes, there are bigger adventures, but not many!
Cheers!