While it's pretty much traditional at this point, there was a very good reason for a long time to do it that way. Gauge for muskets/Fowlers is related to the number of round balls of bore diameter that weights 1 pound, (remember this predates the metric system by about 100+ years). And if you owned a firearm, you had to own the proper mold and cast your own lead balls to shoot over an open campfire. You just couldn't pop down the the sporting goods store and buy some ready made round balls to shoot. Knowing how many round balls per pound lasted into the 1800's. Because if you were a Longhunter or mountain man fur trapper, it was important to know that your .45 caliber Pennsylvania rifle shot 47 to the pound and a .50 caliber Hawken's rifle shot about 35 round ball to the pound. so you could easily know how much lead you needed to bring with over the next year in the lonely mountains.

So if you owned a .72 caliber/12gauge musket you knew you would get 12 round balls per pound of lead. If you had a .69 caliber/14gauge, you would have 14 balls. A 20gauge/.62 caliber will give you 20 round balls.

But yes, the era of the metallic cartridges sent things off the rails in naming cartridges. And post WW1, everyone just completely lost the thread. We have .38 Short, Long, Special, and .357 - all the same bullet diameters. And a seemingly infinite number of .22 caliber cartridges that not even god himself can keep track of.