I was heavily into theater at the time, seeing around 50 performances a year myself, and the idea of seeing a show every night sounded absolutely dismal.
More recently, an actor I like and respect a lot told me, "I don't go see other peoples' shows. I hate going to shows." I was flabbergasted by his honesty. I would imagine a lot of theater people come closer to his viewpoint than the first. Partly because they're tired, busy with their own shows, and want a night or two not spent in a dark windowless room.
Partly also because it's hard watching theatre when you're really into it. Sometimes knowing a lot about a subject makes the experience of being an observer more enriching. Other times it's the opposite dynamic at work.
For me, if the show's not absolutely gripping I find my attention wandering. I'm analyzing the dialogue -- why did the playwright use that turn of phrase? I'm wondering how I'd do in the roles and what I might do differently. I'm attuned to the backstage bumps and the onstage hiccups. I'm picking apart the wardrobe, noting the shoes whose toe points are a decade or two off and the hems that are glued and ironed instead of sewn. And I'm getting ideas for something I might want to write someday.
The next thing you know, 10 minutes have passed and I have no idea what's going on.
Truthfully, most of the shows I've seen have been because I knew someone involved in the production. Sometimes I really wanted to see it, sometimes I did it as a favor, a tit-for-tat thing, and sometimes I went kicking and screaming because I felt like I had to.
In spite of all that, there's always something I enjoy and get out of it, even if it's not entirely a satisfactory experience. And I like seeing and supporting friends. But it's just not my first choice anymore for a night out.