Because there's no quick and easy way to do that on many firearmsWhy we don't universally measure barrel length from the FRONT end of the chamber makes no sense to me!
It's simple on revolvers due to the seperation of the barrel and cylinder
On other action types the simple way is to hook a tape measure to the breech and measure to the end, since barrel and chamber are all inclusive
Those complaining about BBTI's data are pretending those cartridges are only fired in revolvers which have always been measured differently
What they show is no different than a derringer with a very short barrel