Something I've often wondered about.

Perhaps the sign should read “Please reduce your speed to a safe level as you drive your vehicle past this sign.” but I don’t think it would fit.

I think it's an imperative command. A communication shorthand that has facilitated the survival and rapid evolution of our species in recent millennia. Like when we hear a fire alarm it is generally thought to be a good to calmly evacuate the building rather than ruminate on the cause of the fire, who may be to blame, or how hot it is 🔥

Fully agreed, i can be in compliance with the sign if i slowed down 1 km/h out of my inertial speed of 100 km/h

The one I like the best are those found in a school zone which say,

"Slow Children".