i think its because people who really like fish and know what they wanna buy want the satisfaction of owning a certain type of fish or something even if they dont have the tank size, for the moment it works, and eventually they will have to come up with another solution. but for the moment it is satisfying to see that awesome fish you've always wanted swimming around in your tank. at least thats what i would do lol.
im waiting a little longer for mine, a couple of discus, need to set up an extra tank for em. and i have a community too.