It is also that the fish needs room to swim, in other cases have the flow to do so.
While the 1 inch a gallon is as old as people can remember, it is mainly for stores.
It does give you a good indicator if you overstocked or not...
Like putting 2 goldfish in a 5 gallon, if not worse, do 4.
I wish it was as crazy as it sounds but people really do it.
I'm now already looking for a 60/90 gallon...