They make small fish tanks probably because it is cheaper to make them, but they can sell them at a premium price. In stores here, I have seen very small tanks that cost the same amount as a much larger one.
Bettas are quite inactive fish compared to those that live in groups or schools. My guppies were constantly on the move whizzing around their tank, chasing each other and spawning. My splendens on the other hand would usually slowly drift around over the course of the day, maybe stopping to have a rest every now and then.
For me, there is more than stocking a tank than just how many fish will fit in size wise. You also have to take into consideration the activity level requirements of the fish in question. I mean you wouldn't chuck three danios into a 3 gallon tank and they are only the size of guppies.