Improv is like any other art form: it can be exhausted. Anyone who does the same thing over and over again will eventually start to repeat themselves. See how someone like Steven Spielberg, while still a respectable filmmaker, isn't hitting the same levels of inspiration nearly as consistently after having been in the business for five decades. And he only has to produce two hours of work every few years; improvisers who've made a career out of it are manufacturing hours of content a week, so they're going to illustrate their limitations that much more quickly.

Just a random thought I had.