In some version of the myth they actually put an exit back to the living world up there. Sisyphus in life tried to cheat death (even succeeded for a bit) which is part of why he wound up in Tartarus. He could stop pushing the rock any time he wanted, but it would mean giving up on escape.
Well this is what makes it interesting. He has to roll the rock to keep up the lie. But since he does at what point does it stop being a lie? You can ask questions about this and this makes it interesting.