Consistently good Melodists are not usually consistently good Lyricists.
Consistently good Lyricists are not usually consistently good Melodists.
Though there are a great number of exceptions perhaps one of the things that make someone a very good songwriter is recognising that often it takes two?

Good point. Although there are of course some gifted individuals who are able to do both, i.e. come up with good lyrics and melodies, I know at least one lyricist who has no musical knowledge at all, and is tone deaf, but he can write fabulous lyrics. Luckily, he writes in partnership with a talented composer who can write stunning melodies. Between them, they've written some of the best songs I've ever heard.

Needless to say, this pair have never had any great success :-(

To be a successful songwriter is something else entirely, just as being a successful singer or musician is possible without being a good singer or musician.