Why do prog artists peak out?

Why is it so common (and we all know it is) that so many of even the greatest prog bands and composers get better, peak out, then slump in their discography? Wouldn't your compositional skill improve with age?

I ask this because in my studies in music history, most of the classical greats, especially Beethoven, Bach, and Handel, only got better at composing as they got older, and many released their most profound works just before they died.

This seems even true for most of the great jazz artists, like John Coltrane, Miles Davis (before he sold out), and Herbie Hancock, all who continuously put out highly acclaimed works and played with great groups their whole lives.

I'm certainly not trying say these two genres are superior to prog. I'd say prog is anything as diverse and innovative in composition techniques as they are.

But why do prog bands, even the greatest and most commercially successful, burn out overtime, almost seemingly more often than not? Certainly there are some exceptions, but it seems more a general rule to peak at a certain point in the band or composer's career and the several albums after that never get better.

Maybe prog bands tend to have more friction between members? Then why not the jazz groups? Does prog try so hard to be unique and innovative that composers run out of their innovative ideas to fast? It's certainly a relevant and interesting question.