]If your gambling behavior is governed by your greed, this cash-out feature will always be neglected. You always aim for continuous profits and bigger winnings, until you completely forgot that you are gambling, and gambling does not guarantee constant profits and wins, or it could be winnings at first, the rest are massive losses. It’s only when you end up losing all your profits, you start to realized that you are supposed to cash out your profits, and not gamble them all.
I think what happened on @OP is not governed by greed, but rather the person who we are discussing about his action has full confidence in his bet and is probably a person who takes it all or nothing at all. I don't find anything wrong with having confidence in a person's decision about his bet especially when it is in sports betting. It is that luck is not on his side this time and has lost the parlay else, we would be discussing a different outcome scenario.
If one has the full knowledge about the competing team, he surely will have the confidence and will surely decline the offer and see the match/bet till the end.