It is always possible to take any play style to an extreme and Sandbox gaming is no exception.
The extreme sandbox is where a GM prepares everything ahead of time, the players create their characters, the game begins, and the GM referees the players through the setting never ever pushing them in any direction.
This extreme view of Sandbox gaming is unappealing to many GMs as seems to devolve them into being little more than a computer simulating a setting.
Not only this an extreme but it is impossible and not desirable.
It is impossible because you can't make up every detail you will need during a session. At some point you will have to "Make stuff up on the fly."
It is not desirable because of the lopsided wargame scenario problem. There were many time during when I wargamed that either I made a mistake, had bad rolls, or ran into design flaws that made the rest of the scenario a mind numbing grind that I was going to lose. Because many board & counters wargames are complex you couldn't predict when this would happen (except for the design flaw, once you know about it).
Running a Sandbox game as a totally neutral setting will, at some point, lead to grinding sessions that are not fun and feel like a waste of several hours. See my Cow story for one that occurred on my watch as a GM.
However you do have control over your Sandbox Game. You won't be able to control the exact results. But you will be able make sure that the vast majority of time they are interesting and fun results.
There several ways this can happen. First you can control the premises of the campaign. For a crude example a Sandbox based on Judges Guild Wilderlands is different than a sandbox based on Traveller's Spinward Marches. A better example is setting the campaign in City-State versus Viridstan. One is a free-wheeling port of call, the other the imperial
capital to the Wilderland's largest Empire.
Also remember part of the premise isn't just the locales and npcs. Part of it is the events that will occur in the "future". Remember these are the events that will occur if players do NOTHING or never had existed. Once the player start interacting with the setting some of them will chance. Some will not. But having them will give a sense to the players that they are on larger stage. And once they see that they can alter the course of your setting's history then the players will start to invest more into the campaign and be more proactive than reactive.
Next is a realm where players have some control over and that is the premises of their characters. A campaign where the players start out as crusaders from the Church of Mitra will be very different than a campaign where the players are involved with the thieves of the Brotherhood of the Lion.
The hallmark of Sandbox gaming is that the players have the freedom to go where their motivations take them. However the GM also has some option. At every point there is going to a number of possibilities the GM can decide to do in respond to the player's action. Here is where your experience and creativity will come into play. You examine those choices and find the ones that are the most interesting to play out. It doesn't always have to be the most probable as long as it is fun.
Making these choices is really where the GM's creativity shines in a Sandbox Game. It becomes a lot of fun as you go back and forth between yourself and your players.
These are the tools I used to keep my sandbox game interesting and fun. Avoiding another Hunt for the Cow!