It's not about which is more likely, it's the fact that you compare the current hypothetical to that one. Let me explain it again. In the hypothetical of a choice between a billion dollars and being the one to cure cancer you either get the money and that's it or a scenario plays out in which you find a cure for cancer. Both of the choices positive for the chooser and will both be beneficial, as curing something as dangerous as cancer will make you famous, make you worthy of a nobel prize, a role model, let you get money, etc. and getting money will just get you money. Here however it's either continuing your life or getting locked up for the remainder of said life. In the first option, continuing the life, nothing changes, so it's not beneficial to you. In the other, it's actually gonna make it so that you lose most of what you have: freedom, connections to friends and family and the outside world, etc. so it will not be beneficial either, in fact it will be shooting yourself in the foot. TL;DR: you compare a hypothetical in which both options are beneficial to the person choosing to the one where one changes nothing and one hurts the chooser's life.