My answer would be because having it the way have now is the lesser of the two evils.
Any process that has human involvement is prone to error and/or inconsistencies. To eradicate or minimise that as much as possible the process is made prescriptive, and if possible less opinion based. Handball and offside laws are good examples of this.
We have two options here:
1. Intervene if clear and obvious error
2. Intervene if clear and obvious error unless there are unusual circumstances that you think (maybe some considerations) won't require clear and obvious error.
If we take this specific instance out of our head and think about which option serves the game best overall, for me it's option 1, clearly and obviously

.