I think it is very, very difficult to do a sandbox as a published module. Too many possibilities to consider.
Not at all. Open sandbox may seem too wide and broad to be packaged into an adventure, but once you learn to limit the scope of the sandbox to suit your adventure's needs it can be quite manageable.
Another common misconception is that an adventure designed as a sandbox cannot handle any kind of plot or feature villains with motives. A sandbox adventure can certainly have these things without leading players around by the nose or providing only a single trail of breadcrumbs to follow.
A sandbox adventure needs to start with a defined scope. Of course the adventurers can go anywhere they wish but an adventure doesn't have to an entire world or even an entire large region just because of that. The scope could be a small village and surroundings (T1
The Village of Hommlet), or any other specific place. What you decide on for your setting, the scope of the adventure is focused only that part of the setting.
So while there is nothing actually stopping the PCs from heading to Verbobonc while adventuring in Hommlet it is beyond the scope of the adventure to include details for that place in T1. That doesn't mean T1 isn't a sandbox. There are also agents of evil at work in the area that have plans of their own. Within the scope of of the published module, the players can explore and investigate where they wish.
Going a bit further and adding more plot elements to your sandbox, we can look at L2
The Assassins Knot for inspiration. This adventure is an actual murder mystery the PCs need to solve yet it is still a sandbox adventure. The sandbox in this case is a small town and surrounding area. The players begin the adventure with several clues and then enter the sandbox to investigate. Meanwhile the villains behind the foul murder continue with their plans and keep murdering other people! The players may go where they please and when, but the bad guys are operating on a fixed timeline, so the PC's can't just futz about wasting time during the investigation.
So a good published sandbox adventure is not only possible, it's been done before. With a few key points to remember, any decent DM can construct one.
Remember to define the scope of your sandbox for purposes of the adventure.
Determine the resources, strengths & weaknesses of the bad guys.
Outline the ultimate motives, and detail the most immediate plans of these bad guys.
Sketch out a rough timeline of what the outcome of these plans are, should they actually succeed.
Add players and mix well.