I think we have to remember that music is not something that musicians have a copyright on. Music is everyday business for us all if we apply the broadest definition.

Listen to a baby babble, a bird sing, your grandma tell stories, the water in your sink and the wind howling past your ears. All of this has music in it.

A lyricist thinking about a subject, choosing words, structuring meaning units, organizing rhyme patterns etc, is as musical as anything I know.

When there lyrics are enhanced by particular tones, and interpreted on instruments it certainly add to the expression, and becomes more musical as more musical elements are added, but not more musical in terms of better quality. It's just layers of musical expression.

I think it is really great to have these layers, as it is really demanding to write great lyrics. Every layer in a musical production is highly specialized, so a division of work between specialists can really make songs shine IMO

Part of why there are so many lyricists only could be because it is really difficult to be a great lyricist, and that it takes so much effort and focus to reach that level, that other parts of the musical process is put in the background.

From other musicians I hear all the time that they don't feel any good with writing lyrics, but sure they can play the hell out of a song when everything is there. When I was playing, I remember feeling the same (and still not feeling up to par).

Same goes for many lyricists, when they feel competent in writing lyrics, wants to start learning an instrument or take music theory classes. It has always been a good strategy to get good at something first, then you can get some work done and team up with others, while you thereafter develop in other areas.