The short answer is: money. Lots and lots of money, because lyric copyrights will account for up to 50% of the royalties from a tune.
When you're dealing with music sales and (theoretically) getting cuts to artists, lyric authorship & copyright is another piece of the pie, often a substantial one. Essentially, if you have a different person claiming copyright credit for the lyrics, you decrease the royalties earned by the composer of the music part -- the tune, changes, melody, structure.
Look at the credits for a lot of music dating from 1976 to around the mid-90s. Often, as artists developed a career, you will notice that the credited music/lyrics by author names drop from many to just the band member or main artist, whereas in the earliest releases from an artist, you'll many names included in these credits. A standard trick is for industry folks to make "helpful suggestions" that the artist incorporates. Now that suit has become an author, and will get 50% of the royalties from your new hit.