Let’s look at a simple case the 22LR, maximum velocity is achieved somewhere between 16” and 17” after that friction causes the bullet to slow. So in a target rifle 16-17” would be the optimum length for velocity.
With iron sights longer barrels give you more sight radius and in .22lr are quite common. In addition there is a theory that the effect of the exiting gasses is reduced in a longer barrel (26-28”).
Shorter barrels are quite often as or more accurate than the long barrels and this gets into barrel stiffness and harmonics.
Progress the development of smokeless powders and in the mensuration of pressures and velocities has resulted in the ability to achieve targeted velocities in short barrels.
In centerfire rifles, the 20-22” barrel may be a sort of circular thing. Ammunition makers assume that you will have a barrel in this range & so design their ammunition for the length, rifle makers know that ammunition makers design for barrels this length, so they, and ...........you get the idea.
Reloaders more or less go along with this whole scheme caught in the same circular track.

Probably the exception is the benchrest and long range target shooters, but they are operating on what works for others which may be another circular trap.

__________________
Ipsis Rebus Dictantitbus