It's because the lyricist assumes the really specific job of focusing the music, of pointing it somewhere. Words are very sharp objects. On a vocal day, Bono appears with numerous written sheets which he fans out over the floor of the control room. Bono gets singing, jumping physcially and conceptually through the emerging song, weaving lyrical threads into bigger patterns. Meaning is chiselled out bit by bit, polished, broadned, inverted, discarded, revived.
Close attention is paid to subtle shift of vocal tone and emphasis.
Homeless lines wander hopefully from verse to verse. A single ill-fitting word chokes process for half an hour. Flood smokes sympathetically. Work continues in this way until several vocal tracks are recorded. The picture becomes more detailed.