I don't agree often with DU but the point he's making is mostly true. All roleplaying games are sandboxes, there is literally nothing stopping the PC's from doing something completely different or exploring the sandbox except the GM. The GM is the only one who can drag the PC's through the sand to his carefully built sand castle that the PC's are meant to explore. Now mostly the players and GM are in agreement on what content can be explored but if the game takes place in Forgotten Realms then the sandbox is literally the whole universe if the PC's find a means to jump between planes.


The other thing is the illusion of agency. In a traditional RPG where the GM is in control of everything except the PC's, the only agency the players have is over their PC's and even there the only agency is what the GM gives them. There is nothing that stops the GM from invalidating every choice the PC's make which in a sense makes agency an illusion because the only agency the PC's have is what the GM allows them.