Well, that and the fact that most bands suck now. Notice how most of the biggest selling tours are dinosaurs. Elton John, Rolling Stones, KISS, Aerosmith, Journey, Ozzy, McCartney, those are your biggest bands year in and year out.
But I like your point that concerts used to be a huge event. I remember the first time I saw Slayer, I wasn't really sure what the hell they looked like because of their minimal exposure! Imagine seeing Zeppelin or KISS back in the 70's. You are correct, concerts are now something to do, just like the movies or something.
But I think that as long as the "dinosaurs" want to play, people will keep coming. YOu can throw several more acts on top of that. AC/DC, ZZ Top, Sammy Hagar, Skynyrd, Poison, Boston, Billy Joel. These bands will keep making record amounts of money by charging whatever they have to because no one buys their records anymore.