Songwriting

Why does it seem that with most songwriters, they have a period in which they write really good music and then never seem to strike magic again? I think most would agree that DDY and TS did their best work years ago and I think most would say the same for most other bands/songwriters. Rush, Peter Frampton, Kiss, Elton John, REO, Journey, Boston, Kansas, VH and on and on...all continue to make music but in my opinion it seems to always pale in comparison to what they did in their earlier years. I may hear a song now and then that is okay but that is a rarity. I think it's obvious when these bands were young, they were determined to climb that mountain and for whatever reason once they had their run so to speak, they all seemed to peter out. Does one only have so many great songs in them? Once they are successful , are they no longer inspired to write great songs? Is the thrill gone? In my opinion, Styx has not created any music since 1981 that I really really like. Is it partly because I have changed? I find it hard to get that excited about any new music now. I may like something for a short time, but nothing seems to have a lasting effect. I miss that.