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.
We spend so much effort teaching children honesty, respect, to share and to empathise. Then we spend the rest of their childhood beating all that back out of them while telling anyone who wants to cling to those ideals to grow up.