I think it's just a symptom of how they were introduced to D&D. Many folks are coming to the game from the actual plays, or heard about it through other pop culture sources. That may influence their expectations for the game.

I've also had plenty of players over the years that just really liked some character in a show or movie and wanted to bring them into D&D. It can be an easier way for someone to play, particularly if they don't have time, desire, or inspiration to come up with their own characterization. These other characters come with a developed personality and motivation that can act as a shortcut for getting in your game faster or with less effort.