Let me put it this way.
A lone GM working on a setting spends X number of hours writing, creating, doing worldbuilding. Every hour is 1 man hour of work.
A group playing something like Ironsworn (or one of the many, many other games like this) has four or five people sitting around a table constantly world building, writing, creating, meaning that every hour of play is 4 or 5 man hours of work.
That's the advantage to using these systems for doing sandboxes. Say it takes 20 hours of work to get a setting off the ground. I'm picking a totally random number - pick a different one if it works for you. In any case, these systems let you leverage the entire group in sharing the work of building a setting instead of everything funneling through one person at the table.
Even if the players are offering suggestions, it's still going through the GM in order to get it into a working form that can be used at the table.