It is the spacial requirements that would dictate such a large tank, not just volume.
Arrowanas can be safely housed with most large fish that are not top-swimmers (or will not fit in their mouths). Some good choices would be shovelnose or red-tail cats, jaguar cichlids, peacock bass, black belts, pacu, synspilum, payara, mozambique mouthbrooders, clown knife, etc...
As far a people with big tanks in their homes, I know of 5 offhand and could likely find a few more with only a quick phonecall to any of those families. The smallest is 8 x 6 x 4, one is 10 x 4 x 4, another 12 x 3 x 3, yet another 16 x 4 x 4 (WOW!!), and the last 12 x 4 x 4.
I agree these tanks belong in a zoo, but there is no reason why the average person with a healthy budget and dedication to the hobby cannot do it themselves. BTW, some of those people with the big tanks keep average sized aquarium fish. They just like a more natural display, and the effect is stunning! The one that is 6' deep is an incredible example of how fish live in different strata in the wild.
What it boils down to is: know what you are buying, and prepare for it before you buy it. With these fish, by the time you 'get around to it', it has already outgrown your 100 gallon tank. There are a limited number of people with that kind of dedication to their hobby, and when you are trying to get rid of your fish that has outgrown you tank, they already have what they want, and they don't want yours.
/Kris