I understand the tendency to eat a lot during the holidays. But if people are so conscious about weight management, then they should try to eat healthy year-round. (And I know it's hard to diet during the holidays).
If we're talking about losing weight, why even call it a New Year's resolution? It sounds like people are trying to justify losing weight because of the New Year. Well, January 1, 2, 3, or 31 is just a date on the calendar. If people have weight to lose, it doesn't matter if it's Valentine's Day, Easter, the Fourth of July, or Yom Kippur.
That's a bit simplistic, isn't it? Of course people should try to eat healthy year-round. But they don't, for differing reasons, and many people struggle with their weight, eating, and/or exercise habits for much of their life. So they look for a place to make a new start, to definitively break bad habits and form good ones. We all mess up and make mistakes and it's nice to feel like you get another chance at things, which is what New Year's is to some.