The way I see it, it's just like going to see a live sporting event. Why should I go see the Toronto Maple Leafs or Argos, or Jays when it's cheaper to watch it on TV. No traffic, no insane parking fees, no crazy prices for pop and food etc. It's the experience.
The experience is not the same watching it at home. For some it doesn't matter. For others they like the experience, or wouldn't be going. For me, as for movies, some movies need to be experienced at least once on the big screen before watching it on TV. For me, Star Wars, Lord of the Rings need to be watched at a Cinema (since Theaters is for plays and Broadway not movies. ) while others can be watched at home like Romantic comedies.
As for drive ins, I still love going to drive ins. Again it's the experience. It's different watching it in a theatre/cinema and watching it at home.
Movies at Theaters is an up and down. When I was young they were busy then when in my teen age years early advlthood it seemed they were dying, then about 10 years ago took off again and became popular again. Now I think it's on a downhill again.
I understand why popcorn and pop is so much, the theatres only make about 15-25% of movie sales so need to make the money else where. Still I find it crazy to pay for one pop where I can buy a case of it. With a family of 4, it gets really crazy and now just wait for the Blu Rays to come out and buy them then for my collection. Still I still go because the kids like to go (since they don't have to pay for it, it's good, wait till they get a job and want to go see a movie LOL)
So when it's a movie I am really interested in, I will go, to enjoy the experience more since the TV will never compare to the Big Screen.