Increases in ticket prices over the years, due to inflation and other factors, have caused those numbers to go askew from how many people were actually seeing a movie. Headlines touting dollar figures are much more eye-catching than numbers of tickets sold, and it's the sole figure that Hollywood and fandom worships. "Did you hear how much Avatar made this weekend?" is all anyone cares about. Box office records are falling left and right, and the industry took in over ten billion dollars last year.
These days, a film's life or death is charted by how quickly it reaches $100 million at the box office. If it does it very quickly, they'll announce all sorts of sequels and spinoffs are in the works. If it does it slowly, or not at all, you'll seldom hear much more about it unless it becomes a cult hit later on DVD/Blu-ray. But does that number reflect how many people are seeing a movie? It's supposed to be a general benchmark for the popularity of a film ... so why don't we just use numbers of tickets sold instead?