Letting Attacker Live

Hello so genuine question, it comes up quite often that time is running out while the attackers are on a save. Perhaps they just farmed orbs the entire round or just used the eco as a pseudo timeout, but every time this scenario occurs, why do the defenders always run it down to kill them? Wouldn't it be more beneficial if they just let them live?
Is the mental boost of getting a kill just too important in a match, or has the game just not evolved enough that people aren't doing this? It's not that uncommon of a scenario so thought it'd be more common knowledge.

if you go hunt as a defender it's because you don't want the attacker keeping the gun into the next round because they could potentially drop a gun for their teammate + buy one for themselves.
also because you can potentially get an extra ult orb by killing the saving attacker