It doesn't have to be, or have to be present at all.It is a primary one, and I don't think it, or any other reward needs to be used as a lever to get the players to do what the GM wants them to do. That sort of breaks the whole idea of a sandbox, which I had not thought of GM's doing that, and people being against it, even if it makes sense now, that is what is happening.
As to the rest: motivating players to engage the sandbox is the best way to keep it a sandbox. Some players come fully motivated. Others less so. Recognizing what kinds of rewards motivate players and using that knowledge is an important GM skill.