Or they want to share the experience with someone of a younger generation. The first Broadway show I went to was Fiddler on the Roof, in the original production (but not the original cast). I was thrilled to see that there was a revival with Molina, so that my (much) younger cousins could see it. (Not that it had to be on Broadway, but it was important to be prominently available.)
I could add operas, classical plays, and orchestral music. All the schedules for this classifications are dominated by revivals for a long , long time.
Audiences want to play it safe with stuff they know and that figures regularly in the scheduling
Although I don't know if today's 17-year-old would have seen the same thing in Pippin that I did at that age
A lot of the classic musicals are our a key part of American musical legacy, and are great vehicles for today's actors. I think there's something great about passing them down.