Because citizens aren't expected to need to know the scale. They are only expected to follow the guidance their governments issue.
Interpreting this in the worse light possible… in other words, gate keeping.
I think a lot more people could understand and be mature in a lot more things if artificial and archaic barriers weren’t erected. I think not trusting the public is a recipe for an obscure elite ruling class. Sure, not a whole lot of people will get it but at least be open about it.
Then again, maybe there is a benefit to using that particular scale and I’ve been poisoned by my generational ills.
Not trying to be argumentative.
If I hazarded a guess it’s the second reason right?