Pete Townshend said something a few years back to the effect that his early songwriting came from the anger and upheaval of being a young man, and that as a wealthy old man with few relatable problems, he wasn't apt to write any more classics.
Perhaps it's somehow related to the perceived need to do "something new" with rock music. Young musicians strike upon a new sound, or something new to say, and for a few years they run with it, at which point peoples' tastes have changed, leaving the musicians to either continue to do the same thing again and again, or try to rediscover the hunger for newness which got them there in the first place.
I suspect that most people would have trouble coming up with more than 4 or 5 albums's worth of material without starting to repeat themselves.
You said you'd never let me down
But the horse stampedes and rages
In the name of Desperation