Why is it that I want to say "I have theatre tickets" instead of "I have a theatre ticket"? Is it because theatre tickets are rarely purchased as single tickets?

I also always feel like I should dress up (or at least dress nicely) whenever I go to the theatre, ballet, etc. (and no, I don't think of it as a bad thing). I wonder why that is. I know that people used to dress up quite a bit but I've noticed that not many people do now days. Why is that?