Bettas in the wild are also nowhere near as aggressive as their domesticated counterparts. This is why I cannot stand to hear people using the 'but males live together in the wild' argument when attempting to house multiple male splendens together.
Domesticated splendens nowadays tend to look nothing like those in the wild (which sadly are endangered due to hybridisation). They have been bred purely for appearance and aggression for generations now, and really, are only suited for life in an aquarium.
True wild splendens are still aggressive, as are most of the other less popular species of wild bettas. However, they seem to lack the 'hunt down and destroy' drive that most domesticated splendens display.
This is why wilds can usually be kept in pairs or groups. Yes, they still will fight to assert dominance and sort out the pecking order, but their fights are generally only brief, and they don't tend to cause much serious injury.
I had two splendens get in with each other and one of mine was nearly killed. All his pectorals, anal and caudal fin were torn off and his scales were also damaged. Even then, once he had rested, he was still trying to get to the other male and fight.
That one experience is why I would never advise the average owner to house two or more males together, no matter what size the tank.