Kenny G -- why in the world is this man considered a "jazz" artist? He does not improvise, which is a key requirement for any
real jazz performer. It's a shame that that man outsells real jazz musicians.
h2oot -- You're correct on your correlation about singers and material. This is especially true of jazz/adult standards performers, such as Sinatra, Tony Bennett, Ella Fitzgerald and Billie Holiday.
Before the rise of the singer-songwriters in the 1960s and beyond, it wasn't that common for performers to write their own songs.
Noodlin' on a soprano sax
Threaded Mode