It's not that one is opposed to the other. It's quite the contrary. But as far as writing is concerned, writers either write music to existing lyrics (Elton John) or write lyrics to existing melodies and song structures (Paul McCartney). I almost exclusively do the latter. I could write music day and night. The subsequent writing and fitting lyrics is much more difficult. Standard practice: you have to say what you want to say, matching the mood of the music, it usually must rhyme in one scheme or another, and it has to sound right when sung - if you need to hold a note, it has to be a vowel - and obviously, you're constrained by the number of syllables...
As a result, I have probably about 30 unfinished songs - music and melodies with unfinished lyrics.