Yeah, Clapton made quite a career out of covering J.J. Cale’s songs...;-)
Well .. yeah Eric covered a lot of other's songs in his 'younger' days(1). But he sure wasn't alone in that. The Beatles for one did it an awful lot too in their 'Beatlemainia days' and even after that. Ringo doing 'Matchbox' is a cover of Carl Perkins(2) Rockabilly version.
But speaking of covering other's songs, look no further than 'The King'. Elvis covered so many songs, I now wonder if any of his early year stuff was written originally for him(3).
(1) Clapton covered those 'old' and other's songs out of respect and tribute to those 'Greats', not to make the cash register go ding. I'd bet he'd think his best moments came when he was able to play with Blues Legends like BB King and Buddy Guy and other Masters of Blues and R&R.
(2) Go to YouTube and search "Carl Perkins and Friends". Then get ready to sit back and enjoy all the great songs. That is, if you can stay seated and don't start dancing :-)
(3) Sam Phillips really screwed up. IIRC he had Elvis, Carl Perkins, and Jerry Lee Lewis all under contract at the same time.