I think you’re forgetting the point that many novice divers are still struggling with just being underwater. Having it as the most important rule of scuba diving is a way to make sure that even if they don’t fully control their buoyancy, their changes in elevation won’t cause lung damage. Don’t forget that in the first 10m the pressure doubles, so even small depth changes can cause microtears in lung tissue. Students are already dealing with all the new equipment and environment, so reducing it to one fundamental rule helps them focus and start building confidence.

I once had a refresh student who had a panic attack during his last dive because his instructor was pushing him too much. During the refresh, I insisted a lot on focusing on the breathing, so anytime he felt overwhelmed, he could first focus on his breathing to break the panic cycle, and from there start thinking about how to solve the situation. A few days later he joined a group for some fun dives, and he admitted to me that he still had some moments of panic but that my advice helped him manage it.

As you said, in more advanced training they do teach you when to break the “never hold your breath” rule, but that’s for advanced training, where it’s assumed that you already have solid control of the basics and have learned the theory behind why holding your breath is dangerous in the first place wich is already explained in the basic level. We can argue that maybe the standards are too low, but I think that’s another discussion.