The idea of people brewing an amount just because other people do is silly. We all do it our way.
It might be more accurate to say that our equipment choices greatly influence our batch sizes. Once I've invested in tuns/kettles/coolers/kegs that support 5 gallon batches, it's a no-brainer to just brew a 5 gallon batch. Not to mention my recipes are dialed in at 5 gallons, too. I certainly could brew 2 gallons using the same equipment, but there's no big benefit to doing so (for me). I could save a little in ingredients, and maybe some time, but I'd end up with less beer for my efforts. And, being a retired man of leisure, a shorter brew day is not as great a value to me as it may be to some.