The Illusion(?) of Free Will in Sandbox and Role-Playing Video Games

Using both popular examples (e.g. Grand Theft Auto series, Elder Scrolls Series, Minecraft, Skate, Assassin's Creed, etc.) and less popular titles, analyze how games give players the freedom to do as many things as possible with a main mission line looming in the back.
Which games limit features that can only be unlocked by progressing far enough in the main story?
Do some games unlock all features before the main mission is even completed?
Games like Skyrim and Spider-Man (2018) can randomly generate enemy encounters for the player. Is this implemented to give the illusion that there is another world outside the main story? That the player can comfortably abandon the main story since the rest of the game's "world" keeps on going?