When bands start to suck

Why is it that the life cycle of pretty much any decent band/artist is that after about a decade or so they begin to suck?

example:

Metallica when they cut their hair and wrote about their feelings
Led Zeppelin when they went country
System of a Down when they released Hynotised + Mesmerize
Korn after 2004
The Offspring when they released mainstream pop hits
AC/DCs last album which was rather embarassing
Foo Fighters latest bland annoying shit
Ozzy Osbourne after 1998
Gun & Roses new crap + Axel Rose with Botox
Radiohead after Kid A
U2 - after Discotheque - don't get me started on those fags
Rammstein singing about vaginas - IN ENGLISH
Shihad with Beautiful Machine
The Feelers after 1998 - their first album was actually quite good, ie; NOT ANNOYING
Split Enz in the 1980s (apart from Message to my Girl)
Queen in the 1980s
KISS in the 1980s
Sabbath in in 1980s with no Ozzy
Weezer with the Green Album + Beverly Hills
Pretty much anything the Chilli peppers did after 1999 (with the exception of Road Trippin and one or two others)
Gwen Stefani going from punk chick to wigga grrl
Shirlie Manson and Garbage making pop songs played on ZM

All I can think of at the moment.

Exceptions:

The Beatles got better when they discovered psychadelics
Limp Bizkit got better after no one liked them anymore
Pink Floyd got better when Syd Barrett got put in the nut house and replaced
with Waters/Gilmour on lead