This is not a problem unless the monsters intentionally take chances to carry knockouts for whatever reason.
In short, most creatures simply do not benefit from the intentional killing of ground PJs during a fight.
Many enemies are not smart and fight instinctively, without any tactics. If this is the case, then a monster that mortally wounds a player character will want to quickly move the focus to other characters that pose a more immediate threat.
Many enemies are fighting tactically or for any purpose. Once again, the smart option is usually to ignore the injured PC and focus on its allies, which pose an immediate threat. Completing unconscious PCs while there are still enemies alive is to plunder the battlefield before the end of the fight.
Among the sufficiently intelligent, brave and dedicated enemies to kill a PC when the fight is still going on, few of them have the necessary intention. You must 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 that he bleeds to the ground and about twelve seconds from the death. Even in this case, it may be useful to capture live PCs, if possible, such as ransom, bond sale or interrogation for information.