May I speak heresy on this forum? Perhaps a modicum of truth, as well?
As every Betta is different, so is every keeper. For some, having many fish is what brings happiness. In that case financial, but mostly spacial constraints militate against a few large tanks.
For others, building large interesting setups for a few fish is their road to joy. Fishkeeping is often about keeping more than just the fish.
BUT, fishkeeping is for the pleasure of the keeper.
Maintaining healthy active fish enhances that pleasure. The responsibility of every pet owner is to, at minimum, assure just that for his own enjoyment.
If the pleasure of keeping many fish requires smaller accommodations, who is to say the fish resent it? After all, many were grown-out in Jack Daniel pint bottles, shipped in a sandwich bag and displayed in a half-pint cup.
If one has the room for larger tanks and enjoys the look and behavior of their fish in this mode, therein their happiness lies.
The gradations and compromises between these extremes is one among many things that keeps this such a varied and interesting hobby.