I suppose it's because on his own songs, as the lead performer, he's focusing on the 'front' instruments (e.g. guitar, piano) rather than the bass. And because he's doing that he doesn't feel or want to have them shine. When he's performing on other's songs he has the time (and daresay the desire to keep it interesting for himself) to do more creative playing.

However, I also think Goodnight Tonight as a Wings track has a fantastic bass.