Consider that in the wild the loser fish can swim for possibly miles away from the other fish (though probably just a few feet would be adequate as there is likely lots of grass to break up line of vision). When you put a fish in a box, there is simply no place for the losing fish to escape to, thus they will be killed by the victor. So yes, to answer what is often the follow-up question to that, in theory, with a large enough tank, males could be kept together. In fact, we have some very experienced betta keepers who have been able to keep spawns together that contain both males and females. Unfortunately we also have some members that try to do so in small tanks without much experience. :P Also consider that having such a large tank would be stressful for long finned males to try to swim in.
Also, when the land does dry up for the wild fish, if multiple males (and even females) are left in the same puddle there might be some death fights. Sucks, but its nature.
As for a sorority, many people are successful with a 10 gallon sorority. However, a 20 long seems to be ideal. Personally, I think that the success of a sorority is based on how much cover you have. A tank should be incredibly heavily planted if you want a 10 gallon or really any size to be successful. Fake or live plants work, but the benefit of having live is the vastly improved water quality, especially when considering that you should have 4-6 females minimum for a sorority.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?