Thanks, Hawmoon. You put it so well. Heck, they don't even have actual numbers on many aircraft indicators. One glance does it. Can you imagine having to decipher numbers on an artificial horizon indicator?
Except hey do have numbers on most aircraft instruments.
The artificial horizon is not an instrument that needs numbers because of what it is representing as it is a visual indication of the horizon...
Altimeter has numbers
Airspeed indicator has numbers
Rate of climb has numbers
Heading indicator/directional gyro has numbers
Compass has numbers
Engine instruments have numbers
Tachometer and manifold pressure have numbers
Radio navigation instruments have numbers.
Now they do typically indicate most flight instruments on either an analog round type face or a sliding tape scale (think digital flight instruments) for the reasons of quick interpretation and being able to interpret change easily with just a glance. Flight instruments are arranged in a fairly standard configuration for the most part, too.