To be fair, you're not wrong.They both improved other people's music.
That said, Cocker was an interpretive artist (and a bloody great one). He improved music after the event, so to speak. There were others who did this too of course, my own favourite being Aretha.
Bowie didn't really do cover versions (although, most famously with Bing Crosby and Mick Jagger, he was fairly alright at that too). Unlike Cocker, he also made other people's music great in a collaborative way, working brilliantly with the likes of Lou Reed, as mentioned earlier.
I don't think Bowie was remotely unique in this, by the way. Clapton, Dylan, George Harrison, Prince all had (or still have) stellar careers and improved other people's music. Then, there's guys like TBone Burnett who has made just about everyone worth talking about better but has only achieved a middling level of fame.
Moral of the story: music is nothing if not a communal exercise/art form. (And thank God/ Corned beef/ whatever you're having yourself for that).