A lot of them do that, but not all of them. Some may arrange the
music, and re-write the entire lyrics, and just take ownership for
the lyrics, if they've re-written them in whole, and if they only
add in a verse, and re-arrange the rest of the verses, they indicate
that they arranged and adapted the tune, but not take credit for the
whole thing.
Though most of them do claim authorship, there are artists who are
honest.