It is not a problem unless the monsters take an intentional risk to face final blows for some reason.
In short, most creatures simply do not benefit from intentionally killing the downed PCs during the fight.
Many enemies are unintelligent and fight instinctively, regardless of tactics. If that is the case, then a monster that mortally wounds a player character will want to quickly change focus to the other player characters that pose a more immediate threat.
Many enemies fight tactically or with some purpose. Again, the smart choice is usually to ignore the injured PC and focus on its allies, which pose an immediate threat. Ending unconscious PCs while there are still living enemies is like looting the battlefield before the fight ends.
Of the enemies smart enough, brave and dedicated enough to kill a PC when the fight goes on, few have the necessary intention. You would need to be willing to risk your life and that of your allies, just to make sure that a guy dies immediately when he is already unconscious and bleeding on the ground, and anyway about twelve seconds after death. Even so, it can be valuable to capture PC with life if possible, such as rescuing, selling in slavery or interrogating for information.