I will echo what others have already said, and it is based on a distinction between whether you ask what the minimum size is or the perfect size. MINIMUM in my opinion is 2.5, I prefer 5 gallon at least and perfect is obvioulsy the bigger the better. I have a betta in a 5 gallon with a couple endlers, and my dau's is about to be moved from the 2.5 gallon hex into a 10 gallon with some platys.
As far as people using the excuse of "what they are used to", have you seen how bettas are shipped? The guy at the LFS pulled out the box to show me. 100 bettas in a box 12" x 12" x 4" deep. Each one in a plastic bag that is 1/4 if that of a ziploc bag with barely enough water to keep them wet depending how you turn the bag. can they survive like that? Obviously, but don't try to tell me that since it is possible that you can argue 2 ounces of water is "fine"