You must realize that this all started in the 1800s during the Industrial Revolution. There was no agreement as to what measuring system was to be used (Inch? Metric? Other?) so someone tried to standardize things by coming up with wire gauges. The drill sizes followed the wire gauges originally. Now we are stuck with it, just like we are stuck with metric and English measurement systems for everything you can think of, plus really confusing things like bushels and Imperial gallons! The number/letter drill sizes are just another measuring system.
