Some thoughts:

1.Using the word "Cover" shows a bias toward recordings as opposed to live performance

2.Before this singer/songwriter recording craze, songwriters wrote mostly with the intent of having others perform their music--Rodgers and Hammerstein never had any hits of their own songs

3.Irma Thomas did the first record that I ever heard of "Yime is on My Side" and still the best--

4. Patsy Cline recorded "Crazy" years before Willie Nelson did--

5. A lot of songwriters aren't particularly good singers

6. A lot of the singer/songwriter type of material tends to be fairly personal, or ideosyncratic, so that it doesn't have much appeal to anyone other than the writer--

7. A lot of singer/songwriters don't do other material well--

Present company excepted--