Reference:
Apart from Elvis What about ...Frank Sinatra, Shirley Bassey, Jimmy Hendrix, Rihanna, Joe Cocker, Three Dog Night, Nina Simone, Barbra Streisand and so many more...I don't think it matters....there are singers who can't write and writers who can't sing.
The thing is that the music industry has changed enormously, and public attitudes have changed as a result.
It used to be that singers were just that - singers. As has been said elsewhere on this thread, the human voice was treated as an instrument, and appropriate singers would be matched up with appropriate songs.
The quality of the singer's voice was often as important - if not more
important - than the song itself. I remember seeing a reproduction of an old NME front cover from the 50's (back in the days when they printed stories on the front page) which was given over to a "right of reply" from Frank Sinatra. Sinatra was touring Europe at the time, and there had been some press criticism of his diction and general vocal quality. Old Blue Eyes was not happy, and defended himself in no uncertain terms. Can you imagine a similar fuss nowadays over the vocal ability of, say, Lady Gaga? It's much more likely that any controversy would be over a major singer
miming!
This is also all tied up with a changed understanding of what "cover versions" are for. Nowadays, cover versions are an easy way for artists (even those who
do write their own material) to break through, because a known song gives them instant recognition with the public. However it used to be that when a song was written, several cover versions would be released
at the same time. The idea was people would buy the version by the singer they preferred.
A good example is "Unchained Melody". Most people associate that song with the Righteous Brothers, but their recording came out in 1965 and was an example of the modern cover version. The song
originally came out in 1955, and in that year alone there were
four different versions of the song (both vocal and instrumental) in the UK top 20 - the most successful being by a certain Jimmy Young...
Arguably, the whole notion of bands writing their own material goes back to The Beatles, but even Lennon and McCartney often went down the "Tin Pan Alley" route and wrote songs for other singers such as Cilla Black...