Well it's true that you are not supposed to feel regret during such case but believe me you would definitely feel regret when it happens to you and I say this because I had same feeling with one of my bet which was remaining sporting cp to win and they had just some minutes left to play of which I was offered cash out but like you said you would definitely not be tempted to take it since they are already winning but at the very dying minutes they equalized them.
To be honest, you’re right - no matter how little or far you were from winning you’ll still have that feeling of regret; the “had I known I would have just settled for the little cash that was available for me instead of having to go with nothing”.
But the truth is that you can’t predict if that last game is going to cut everything. - imagine you bet on home to win in a football match and at the 87th minute Home was leading with 1 goal and then the casino gives you the cashout option I’m sure majority of us here will let the game end, but even in that game it’s possible for Away to score an equalizer and make you lose everything. If something like this happens I’m sure everyone would feel bad but not as much as when they gave you the option in game you were not even sure of right from when you selected it but still decide to ignore the cash out option.