The core standard
Referees are trained to judge whether a fighter can intelligently defend themselves, a standard that is broader than simply whether the fighter is conscious. A fighter can be conscious but unable to protect themselves in a coherent way, covering up without purpose or failing to react to strikes at all, and that state is what a referee is watching for far more than any single dramatic blow.
Position and momentum matter
A fighter trapped against the cage or on the ground under a series of unanswered strikes is treated differently from one exchanging blows in open space, because escape options and the ability to recover position differ sharply between those situations. Referees weigh not just the strikes landed but whether the fighter in trouble has any realistic path to improving their position before the next strike arrives.
The tension that never resolves
Stop a fight too early and a fighter who could have recovered and won is denied that chance, which damages their record and can affect their career. Stop it too late and a fighter absorbs damage that a timelier stoppage would have prevented. There is no rule that eliminates this tension entirely, which is why referees are judged individually on their reads rather than against a fixed checklist that could remove all ambiguity.